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/group.go

@@ -12,30 +12,32 @@ import (
 
 // Ensure the struct implements the interface.
 var (
-	_ Auth = &Group{}
+	_ Method        = &Group{}
+	_ Initializable = &Group{}
+	_ Freeable      = &Group{}
 )
 
 // Group implements the Auth interface with serval Auth.
 type Group struct {
-	methods []Auth
+	methods []Method
 }
 
 // NewGroup creates a new auth group
-func NewGroup(methods ...Auth) *Group {
+func NewGroup(methods ...Method) *Group {
 	return &Group{
 		methods: methods,
 	}
 }
 
-// Name represents the name of auth method
-func (b *Group) Name() string {
-	return "group"
-}
-
 // Init does nothing as the Basic implementation does not need to allocate any resources
 func (b *Group) Init() error {
-	for _, m := range b.methods {
-		if err := m.Init(); err != nil {
+	for _, method := range b.methods {
+		initializable, ok := method.(Initializable)
+		if !ok {
+			continue
+		}
+
+		if err := initializable.Init(); err != nil {
 			return err
 		}
 	}
@@ -44,8 +46,12 @@ func (b *Group) Init() error {
 
 // Free does nothing as the Basic implementation does not have to release any resources
 func (b *Group) Free() error {
-	for _, m := range b.methods {
-		if err := m.Free(); err != nil {
+	for _, method := range b.methods {
+		freeable, ok := method.(Freeable)
+		if !ok {
+			continue
+		}
+		if err := freeable.Free(); err != nil {
 			return err
 		}
 	}
@@ -63,7 +69,9 @@ func (b *Group) Verify(req *http.Request, w http.ResponseWriter, store DataStore
 		user := ssoMethod.Verify(req, w, store, sess)
 		if user != nil {
 			if store.GetData()["AuthedMethod"] == nil {
-				store.GetData()["AuthedMethod"] = ssoMethod.Name()
+				if named, ok := ssoMethod.(Named); ok {
+					store.GetData()["AuthedMethod"] = named.Name()
+				}
 			}
 			return user
 		}