Allow only internal registration (#15795)

* Add ALLOW_ONLY_INTERNAL_REGISTRATION into settings

* OpenID respect setting too

options/locale/locale_en-US.ini

@@ -2412,6 +2412,7 @@ config.db_path = Path
 config.service_config = Service Configuration
 config.register_email_confirm = Require Email Confirmation to Register
 config.disable_register = Disable Self-Registration
+config.allow_only_internal_registration = Allow Registration Only Through Gitea itself
 config.allow_only_external_registration = Allow Registration Only Through External Services
 config.enable_openid_signup = Enable OpenID Self-Registration
 config.enable_openid_signin = Enable OpenID Sign-In