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/communities_controller.rb

@@ -8,6 +8,7 @@ class CommunitiesController < ApplicationController
 
   def show
     raise ActionController::RoutingError, "Not Found" unless communitable_exists?
+
     redirect_to root_path if Setting["feature.community"].blank?
   end