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>

routers/web/user/oauth.go

@@ -13,7 +13,6 @@ import (
 	"strings"
 
 	"code.gitea.io/gitea/models"
-	"code.gitea.io/gitea/modules/auth/oauth2"
 	"code.gitea.io/gitea/modules/base"
 	"code.gitea.io/gitea/modules/context"
 	"code.gitea.io/gitea/modules/log"
@@ -21,6 +20,7 @@ import (
 	"code.gitea.io/gitea/modules/timeutil"
 	"code.gitea.io/gitea/modules/web"
 	"code.gitea.io/gitea/services/auth"
+	"code.gitea.io/gitea/services/auth/source/oauth2"
 	"code.gitea.io/gitea/services/forms"
 
 	"gitea.com/go-chi/binding"
@@ -144,9 +144,9 @@ func newAccessTokenResponse(grant *models.OAuth2Grant, signingKey oauth2.JWTSign
 	}
 	// generate access token to access the API
 	expirationDate := timeutil.TimeStampNow().Add(setting.OAuth2.AccessTokenExpirationTime)
-	accessToken := &models.OAuth2Token{
+	accessToken := &oauth2.Token{
 		GrantID: grant.ID,
-		Type:    models.TypeAccessToken,
+		Type:    oauth2.TypeAccessToken,
 		StandardClaims: jwt.StandardClaims{
 			ExpiresAt: expirationDate.AsTime().Unix(),
 		},
@@ -161,10 +161,10 @@ func newAccessTokenResponse(grant *models.OAuth2Grant, signingKey oauth2.JWTSign
 
 	// generate refresh token to request an access token after it expired later
 	refreshExpirationDate := timeutil.TimeStampNow().Add(setting.OAuth2.RefreshTokenExpirationTime * 60 * 60).AsTime().Unix()
-	refreshToken := &models.OAuth2Token{
+	refreshToken := &oauth2.Token{
 		GrantID: grant.ID,
 		Counter: grant.Counter,
-		Type:    models.TypeRefreshToken,
+		Type:    oauth2.TypeRefreshToken,
 		StandardClaims: jwt.StandardClaims{
 			ExpiresAt: refreshExpirationDate,
 		},
@@ -202,7 +202,7 @@ func newAccessTokenResponse(grant *models.OAuth2Grant, signingKey oauth2.JWTSign
 			}
 		}
 
-		idToken := &models.OIDCToken{
+		idToken := &oauth2.OIDCToken{
 			StandardClaims: jwt.StandardClaims{
 				ExpiresAt: expirationDate.AsTime().Unix(),
 				Issuer:    setting.AppURL,
@@ -568,7 +568,7 @@ func AccessTokenOAuth(ctx *context.Context) {
 }
 
 func handleRefreshToken(ctx *context.Context, form forms.AccessTokenForm, signingKey oauth2.JWTSigningKey) {
-	token, err := models.ParseOAuth2Token(form.RefreshToken)
+	token, err := oauth2.ParseToken(form.RefreshToken)
 	if err != nil {
 		handleAccessTokenError(ctx, AccessTokenError{
 			ErrorCode:        AccessTokenErrorCodeUnauthorizedClient,