Add cucumber tests for basic search functionality

Some of our document formats (AAIB and Drug Safety Updates, for
example) have many documents, and our publishing tool doesn't easily
allow users to find a document to amend other than by paging through the
results.

We should allow users to search by either slug or title using a form on
the list page.
Add 'reform' gem to represent search form

In order to use the rails form helpers, and get niceties such as the
form reflecting the model during the search process, we need the object
storing the form parameters to conform to an ActiveModel style API.

By adding `reform`, we have the ability to do this without backing it
with a persisted model.

In specialist-publisher, we used to do this with app/view_adapters, but
these have become so customised to specialist documents that creating a
new view adapter would be overcomplex for this requirement.
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.
Merge pull request #286 from alphagov/add_search_form

Allow searching Specialist Documents by Slug and Title