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/auth.go

@@ -27,7 +27,7 @@ import (
 //
 // The Session plugin is expected to be executed second, in order to skip authentication
 // for users that have already signed in.
-var authMethods = []Auth{
+var authMethods = []Method{
 	&OAuth2{},
 	&Basic{},
 	&Session{},
@@ -40,12 +40,12 @@ var (
 )
 
 // Methods returns the instances of all registered methods
-func Methods() []Auth {
+func Methods() []Method {
 	return authMethods
 }
 
 // Register adds the specified instance to the list of available methods
-func Register(method Auth) {
+func Register(method Method) {
 	authMethods = append(authMethods, method)
 }
 
@@ -57,7 +57,12 @@ func Init() {
 	}
 	specialInit()
 	for _, method := range Methods() {
-		err := method.Init()
+		initializable, ok := method.(Initializable)
+		if !ok {
+			continue
+		}
+
+		err := initializable.Init()
 		if err != nil {
 			log.Error("Could not initialize '%s' auth method, error: %s", reflect.TypeOf(method).String(), err)
 		}
@@ -68,7 +73,12 @@ func Init() {
 // to release necessary resources
 func Free() {
 	for _, method := range Methods() {
-		err := method.Free()
+		freeable, ok := method.(Freeable)
+		if !ok {
+			continue
+		}
+
+		err := freeable.Free()
 		if err != nil {
 			log.Error("Could not free '%s' auth method, error: %s", reflect.TypeOf(method).String(), err)
 		}