feat: Add option to disable builtin authentication.

Setting ENABLE_INTERNAL_SIGNIN to false will disable the built-in
signin form, should the administrator prefer to limit users to SSO.

Continuation of forgejo/forgejo#6076

custom/conf/app.example.ini

@@ -901,6 +901,9 @@ LEVEL = Info
 ;; Show Registration button
 ;SHOW_REGISTRATION_BUTTON = true
 ;;
+;; Whether to allow internal signin
+; ENABLE_INTERNAL_SIGNIN = true
+;;
 ;; Show milestones dashboard page - a view of all the user's milestones
 ;SHOW_MILESTONES_DASHBOARD_PAGE = true
 ;;