Minimize CE/EE difference in Gitlab::Auth::UserAuthFinders

Signed-off-by: NRémy Coutable <remy@rymai.me>

lib/gitlab/auth/user_auth_finders.rb

 module Gitlab
   module Auth
-    #
-    # Exceptions
-    #
-
     AuthenticationError = Class.new(StandardError)
     MissingTokenError = Class.new(AuthenticationError)
     TokenNotFoundError = Class.new(AuthenticationError)
@@ -61,6 +57,12 @@ module Gitlab
 
       private
 
+      def route_authentication_setting
+        return {} unless respond_to?(:route_setting)
+
+        route_setting(:authentication) || {}
+      end
+
       def access_token
         strong_memoize(:access_token) do
           find_oauth_access_token || find_personal_access_token