Allow users to search for documents by slug or title

* Display a search form (partial) on the index page for all specialist
  documents

* Modify the ListDocumentsService to accept a search_details argument
  describing parameters the user searches for

* If the user is searching rather than listing, truncate the number of
  results to the kaminari pagination limit. This is because doing
  anything else would require modifying the paginator to paginate the
  sorted collection, which isn't straightforward. The limitation this
  adds (cannot view more than the top 50 results of a search) has been
  cleared with the PO.
Fix flash message classes for bootstrap 3

Bootstrap 3 changes flash classes to `alert alert-warning` from `alert
warning`