Refactor: Move login out of models (#16199)

`models` does far too much. In particular it handles all `UserSignin`.

It shouldn't be responsible for calling LDAP, SMTP or PAM for signing in.

Therefore we should move this code out of `models`.

This code has to depend on `models` - therefore it belongs in `services`.

There is a package in `services` called `auth` and clearly this functionality belongs in there.

Plan:

- [x] Change `auth.Auth` to `auth.Method` - as they represent methods of authentication.
- [x] Move `models.UserSignIn` into `auth`
- [x] Move `models.ExternalUserLogin`
- [x] Move most of the `LoginVia*` methods to `auth` or subpackages
- [x] Move Resynchronize functionality to `auth`
  - Involved some restructuring of `models/ssh_key.go` to reduce the size of this massive file and simplify its files.
- [x] Move the rest of the LDAP functionality in to the ldap subpackage
- [x] Re-factor the login sources to express an interfaces `auth.Source`?
  - I've done this through some smaller interfaces Authenticator and Synchronizable - which would allow us to extend things in future
- [x] Now LDAP is out of models - need to think about modules/auth/ldap and I think all of that functionality might just be moveable
- [x] Similarly a lot Oauth2 functionality need not be in models too and should be moved to services/auth/source/oauth2
  - [x] modules/auth/oauth2/oauth2.go uses xorm... This is naughty - probably need to move this into models.
  - [x] models/oauth2.go - mostly should be in modules/auth/oauth2 or services/auth/source/oauth2 
- [x] More simplifications of login_source.go may need to be done
- Allow wiring in of notify registration -  *this can now easily be done - but I think we should do it in another PR*  - see #16178 
- More refactors...?
  - OpenID should probably become an auth Method but I think that can be left for another PR
  - Methods should also probably be cleaned up  - again another PR I think.
  - SSPI still needs more refactors.* Rename auth.Auth auth.Method
* Restructure ssh_key.go

- move functions from models/user.go that relate to ssh_key to ssh_key
- split ssh_key.go to try create clearer function domains for allow for
future refactors here.

Signed-off-by: Andrew Thornton <art27@cantab.net>

services/auth/source/ldap/source.go

@@ -0,0 +1,120 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package ldap
+
+import (
+	"strings"
+
+	"code.gitea.io/gitea/models"
+	"code.gitea.io/gitea/modules/secret"
+	"code.gitea.io/gitea/modules/setting"
+
+	jsoniter "github.com/json-iterator/go"
+)
+
+// .____     ________      _____ __________
+// |    |    \______ \    /  _  \\______   \
+// |    |     |    |  \  /  /_\  \|     ___/
+// |    |___  |    `   \/    |    \    |
+// |_______ \/_______  /\____|__  /____|
+//         \/        \/         \/
+
+// Package ldap provide functions & structure to query a LDAP ldap directory
+// For now, it's mainly tested again an MS Active Directory service, see README.md for more information
+
+// Source Basic LDAP authentication service
+type Source struct {
+	Name                  string // canonical name (ie. corporate.ad)
+	Host                  string // LDAP host
+	Port                  int    // port number
+	SecurityProtocol      SecurityProtocol
+	SkipVerify            bool
+	BindDN                string // DN to bind with
+	BindPasswordEncrypt   string // Encrypted Bind BN password
+	BindPassword          string // Bind DN password
+	UserBase              string // Base search path for users
+	UserDN                string // Template for the DN of the user for simple auth
+	AttributeUsername     string // Username attribute
+	AttributeName         string // First name attribute
+	AttributeSurname      string // Surname attribute
+	AttributeMail         string // E-mail attribute
+	AttributesInBind      bool   // fetch attributes in bind context (not user)
+	AttributeSSHPublicKey string // LDAP SSH Public Key attribute
+	SearchPageSize        uint32 // Search with paging page size
+	Filter                string // Query filter to validate entry
+	AdminFilter           string // Query filter to check if user is admin
+	RestrictedFilter      string // Query filter to check if user is restricted
+	Enabled               bool   // if this source is disabled
+	AllowDeactivateAll    bool   // Allow an empty search response to deactivate all users from this source
+	GroupsEnabled         bool   // if the group checking is enabled
+	GroupDN               string // Group Search Base
+	GroupFilter           string // Group Name Filter
+	GroupMemberUID        string // Group Attribute containing array of UserUID
+	UserUID               string // User Attribute listed in Group
+
+	// reference to the loginSource
+	loginSource *models.LoginSource
+}
+
+// FromDB fills up a LDAPConfig from serialized format.
+func (source *Source) FromDB(bs []byte) error {
+	err := models.JSONUnmarshalHandleDoubleEncode(bs, &source)
+	if err != nil {
+		return err
+	}
+	if source.BindPasswordEncrypt != "" {
+		source.BindPassword, err = secret.DecryptSecret(setting.SecretKey, source.BindPasswordEncrypt)
+		source.BindPasswordEncrypt = ""
+	}
+	return err
+}
+
+// ToDB exports a LDAPConfig to a serialized format.
+func (source *Source) ToDB() ([]byte, error) {
+	var err error
+	source.BindPasswordEncrypt, err = secret.EncryptSecret(setting.SecretKey, source.BindPassword)
+	if err != nil {
+		return nil, err
+	}
+	source.BindPassword = ""
+	json := jsoniter.ConfigCompatibleWithStandardLibrary
+	return json.Marshal(source)
+}
+
+// SecurityProtocolName returns the name of configured security
+// protocol.
+func (source *Source) SecurityProtocolName() string {
+	return SecurityProtocolNames[source.SecurityProtocol]
+}
+
+// IsSkipVerify returns if SkipVerify is set
+func (source *Source) IsSkipVerify() bool {
+	return source.SkipVerify
+}
+
+// HasTLS returns if HasTLS
+func (source *Source) HasTLS() bool {
+	return source.SecurityProtocol > SecurityProtocolUnencrypted
+}
+
+// UseTLS returns if UseTLS
+func (source *Source) UseTLS() bool {
+	return source.SecurityProtocol != SecurityProtocolUnencrypted
+}
+
+// ProvidesSSHKeys returns if this source provides SSH Keys
+func (source *Source) ProvidesSSHKeys() bool {
+	return len(strings.TrimSpace(source.AttributeSSHPublicKey)) > 0
+}
+
+// SetLoginSource sets the related LoginSource
+func (source *Source) SetLoginSource(loginSource *models.LoginSource) {
+	source.loginSource = loginSource
+}
+
+func init() {
+	models.RegisterLoginTypeConfig(models.LoginLDAP, &Source{})
+	models.RegisterLoginTypeConfig(models.LoginDLDAP, &Source{})
+}