Adding tests for filtering.

Previously there didn't seem to be any tests covering the
filters for publications, policies and announcements.  This
commit adds cucumber features covering most of their functionality.
Don't include rendered mustache templates

Rendered mustache templates had been added to the repository because a JS
cucumber feature needed them. This was bad.

This commit removes the rendered templates and tells cucumber to render
them on demand instead.
Don't render mustache templates in cucumber env

It caused problems on CI
Improve wording of cucumber scenarios
Merge pull request #980 from alphagov/add-tests-for-document-filter

Adding tests for filtering