Add and apply EmptyLineAfterGuardClause rule

We were inconsistent on this one. I consider it particularly useful when
a method starts with a `return` statement.

In other cases, we probably shouldn't have a guard rule in the middle of
a method in any case, but that's a different refactoring.

app/controllers/admin/settings_controller.rb

@@ -55,6 +55,7 @@ class Admin::SettingsController < Admin::BaseController
 
     def request_referer
       return request.referer + params[:setting][:tab] if params[:setting][:tab]
+
       request.referer
     end
 end