use a nil subject when we want to check global abilities

lib/api/groups.rb

@@ -30,7 +30,7 @@ module API
       # Example Request:
       #   POST /groups
       post do
-        authorize! :create_group, current_user
+        authorize! :create_group
         required_attributes! [:name, :path]
         attrs = attributes_for_keys [:name, :path, :description, :visibility_level]

lib/api/helpers.rb

@@ -129,7 +129,7 @@ module API
       forbidden! unless current_user.is_admin?
     end
-    def authorize!(action, subject)
+    def authorize!(action, subject = nil)
       forbidden! unless can?(current_user, action, subject)
     end