Adding in snippet search functionality

Created by: bushong1

This is a resubmit of the closed PR#7612. While fixing some things, a refactor of search was pushed to master. This PR is updating snippet-search for the refactor.

In reference to this merge request:

http://feedback.gitlab.com/forums/176466-general/suggestions/5529795-search-though-snippets

What does this MR do?

Adds the capability to search snippets. This can be accessed whenever viewing a snippet related page, such as personal snippets, snippet discovery, and search snippets. There is a tab for searching just filenames and titles, and there is a tab for searching just the code of the snippet. It searches all public snippets, as well as the private snippets of the user.

Are there points in the code the reviewer needs to double check?

The largest change outside of haml files is lib/gitlab/snippet_search_results.rb.

Why was this MR needed?

This is something my users have been asking for, and it should add a great amount of collaborative capability and global discovery to the snippets feature. In addition, there is a merge request already created with over 20 points.

What are the relevant issue numbers / Feature requests?

http://feedback.gitlab.com/forums/176466-general/suggestions/5529795-search-though-snippets

Screenshots (If appropiate)