Take search query into search bar on 404

• Follow-up from https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/345
• For https://gitlab.com/gitlab-com/gitlab-docs/issues/246

What

Take the search query from the URL the user is coming from into the search input field when landing on the 404 page

How

Suggestion: something like this:

• get the URL the user is coming from
• strip the URL, like whatever/hello_world_search_query.html to "hello world search query"
• take the stripped query ("hello world search query") into the search input field and load the search results normally

cc/ @mikelewis FYI

@iamphill do you know how to do this?