License plan check on namespaces

Addresses #2019

• Create helper methods for checking feature availability for namespaces
• Put 'File locks' and 'Deploy board' under namespace license check
• Allow checking features by using symbols instead plain long strings (e.g. :file_lock, :deploy_board)
• Add namespace API for admin usage

app/controllers/admin/application_settings_controller.rb

@@ -174,7 +174,8 @@ class Admin::ApplicationSettingsController < Admin::ApplicationController
       :shared_runners_minutes,
       :minimum_mirror_sync_time,
       :geo_status_timeout,
-      :elasticsearch_experimental_indexer
+      :elasticsearch_experimental_indexer,
+      :check_namespace_plan
     ]
   end
 end