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.
Initial work on searching
Fix flash message classes for bootstrap 3

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