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/concerns/search.rb

@@ -18,6 +18,7 @@ module Search
 
   def parse_search_date
     return unless search_by_date?
+
     params[:advanced_search][:date_range] = search_date_range
   end