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/oauth2.go

@@ -14,11 +14,13 @@ import (
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/timeutil"
 	"code.gitea.io/gitea/modules/web/middleware"
+	"code.gitea.io/gitea/services/auth/source/oauth2"
 )
 
 // Ensure the struct implements the interface.
 var (
-	_ Auth = &OAuth2{}
+	_ Method = &OAuth2{}
+	_ Named  = &OAuth2{}
 )
 
 // CheckOAuthAccessToken returns uid of user from oauth token
@@ -27,7 +29,7 @@ func CheckOAuthAccessToken(accessToken string) int64 {
 	if !strings.Contains(accessToken, ".") {
 		return 0
 	}
-	token, err := models.ParseOAuth2Token(accessToken)
+	token, err := oauth2.ParseToken(accessToken)
 	if err != nil {
 		log.Trace("ParseOAuth2Token: %v", err)
 		return 0
@@ -36,7 +38,7 @@ func CheckOAuthAccessToken(accessToken string) int64 {
 	if grant, err = models.GetOAuth2GrantByID(token.GrantID); err != nil || grant == nil {
 		return 0
 	}
-	if token.Type != models.TypeAccessToken {
+	if token.Type != oauth2.TypeAccessToken {
 		return 0
 	}
 	if token.ExpiresAt < time.Now().Unix() || token.IssuedAt > time.Now().Unix() {
@@ -51,21 +53,11 @@ func CheckOAuthAccessToken(accessToken string) int64 {
 type OAuth2 struct {
 }
 
-// Init does nothing as the OAuth2 implementation does not need to allocate any resources
-func (o *OAuth2) Init() error {
-	return nil
-}
-
 // Name represents the name of auth method
 func (o *OAuth2) Name() string {
 	return "oauth2"
 }
 
-// Free does nothing as the OAuth2 implementation does not have to release any resources
-func (o *OAuth2) Free() error {
-	return nil
-}
-
 // userIDFromToken returns the user id corresponding to the OAuth token.
 func (o *OAuth2) userIDFromToken(req *http.Request, store DataStore) int64 {
 	_ = req.ParseForm()