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/en.json

@@ -1560,7 +1560,7 @@
     "translation": "Revoking sessionId=%v for userId=%v re-login with same device Id"
   },
   {
-    "id": "api.user.login_by_email.sign_in.app_error",
+    "id": "api.user.login.use_auth_service.app_error",
     "translation": "Please sign in using {{.AuthService}}"
   },
   {
@@ -3443,6 +3443,14 @@
     "id": "store.sql_user.get_for_export.app_error",
     "translation": "We encountered an error while finding user profiles"
   },
+  {
+    "id": "store.sql_user.get_for_login.app_error",
+    "translation": "We couldn't find an existing account matching your credentials. This team may require an invite from the team owner to join."
+  },
+  {
+    "id": "store.sql_user.get_for_login.multiple_users",
+    "translation": "We found multiple users matching your credentials and were unable to log you in. Please contact an administrator."
+  },
   {
     "id": "store.sql_user.get_profiles.app_error",
     "translation": "We encountered an error while finding user profiles"

i18n/es.json

@@ -1560,7 +1560,7 @@
     "translation": "Revocando sessionId=%v para el userId=%v vuelve a iniciar la sesión el el mismo dispositivo"
   },
   {
-    "id": "api.user.login_by_email.sign_in.app_error",
+    "id": "api.user.login.use_auth_service.app_error",
     "translation": "Por favor inicia sesión usando {{.AuthService}}"
   },
   {

i18n/fr.json

@@ -1428,7 +1428,7 @@
     "translation": "Session sessionId=%v de l'utilisateur userId=%v révoquée re-connexion avec le même identifiant"
   },
   {
-    "id": "api.user.login_by_email.sign_in.app_error",
+    "id": "api.user.login.use_auth_service.app_error",
     "translation": "Veuillez vous connecter en utilisant  {{.AuthService}}"
   },
   {

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"
   },
   {

i18n/pt.json

@@ -1520,7 +1520,7 @@
     "translation": "Revogada sessionid=%v para userid=%v logue novamente com o mesmo id de dispositivo"
   },
   {
-    "id": "api.user.login_by_email.sign_in.app_error",
+    "id": "api.user.login.use_auth_service.app_error",
     "translation": "Por favor logue usando {{.AuthService}}"
   },
   {