PLT-2258 Unified login screen and related APIs (#2820)

* Unified login screen and related APIs

* Refactored login API call to be less convoluted

* Removed LDAP login prompt from invite process

* Fixed existing LDAP users being able to log in if LDAP was configured, but disabled

* Gofmt

* Future proofed login API

* Updated login APIs based on feedback

* Added additional auditing to login API

* Actually removed loginById

i18n/ja.json

@@ -1564,7 +1564,7 @@
     "translation": "userId=%vのsessionId=%vを無効化しました。同じデバイスIDでログインし直してください"
   },
   {
-    "id": "api.user.login_by_email.sign_in.app_error",
+    "id": "api.user.login.use_auth_service.app_error",
     "translation": "{{.AuthService}}を使ってサインインしてください"
   },
   {