Pull out separate model for announcement date

This is the first step towards a model where the release date can be
changed and logged with an audit trail.
Remove unused cucumber step file
Rework document finder

This is so that a separate form can be used for the document-finder
and the thing that is using it.
Rework stats announcements workflow

The first release will be for ONS users only whilst we road-test
things so listing announcements on a user's dashboard won't make sense
in the short term. This adds a statistics announcement index page and
updates the creation and editing of announcements to work around it.
It also adds a show page to accomodate the extra workflows that users
will perform on announcements.

Still to come is the functionality that allows users to change the
announced release date.