Adding query string to search form

Fixes #3426 . Adding the query string used in the 404 search to the text field in the search form displayed on the 404 page. 

@aaronschachter @deadlybutter 
@aaronschachter hmmm, no i'm not seeing any errors or warnings. Maybe because I am only passing in the query string argument when rendering the form on the 404 page. But I can do a double check for the existence of the query string to be more defensive.