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/basic.go

@@ -19,7 +19,8 @@ import (
 
 // Ensure the struct implements the interface.
 var (
-	_ Auth = &Basic{}
+	_ Method = &Basic{}
+	_ Named  = &Basic{}
 )
 
 // Basic implements the Auth interface and authenticates requests (API requests
@@ -33,16 +34,6 @@ func (b *Basic) Name() string {
 	return "basic"
 }
 
-// Init does nothing as the Basic implementation does not need to allocate any resources
-func (b *Basic) Init() error {
-	return nil
-}
-
-// Free does nothing as the Basic implementation does not have to release any resources
-func (b *Basic) Free() error {
-	return nil
-}
-
 // Verify extracts and validates Basic data (username and password/token) from the
 // "Authorization" header of the request and returns the corresponding user object for that
 // name/token on successful validation.
@@ -116,7 +107,7 @@ func (b *Basic) Verify(req *http.Request, w http.ResponseWriter, store DataStore
 	}
 
 	log.Trace("Basic Authorization: Attempting SignIn for %s", uname)
-	u, err := models.UserSignIn(uname, passwd)
+	u, err := UserSignIn(uname, passwd)
 	if err != nil {
 		if !models.IsErrUserNotExist(err) {
 			log.Error("UserSignIn: %v", err)