Frontend part of announceable statistics

This PR is a combination of both the publisher side and the citizen side of statistics announcements.

Notes on the frontend part:

Statistics announcements have a frontend representation called Frontend::StatisticsAnnouncement.  These are provided by a service class called Frontend::StatisticsAnnouncementProvider.  The provider includes a class which mimics the behaviour of Rummager using SQL for dev/test mode.

The filter for statistics announcements is driven entirely by new objects.  I have looked into what could be re-used from the document filter, but that code was rather hairy and extremely specific to documents.  I figured that it would be better to repeat a bit of functionality than to couple this new code area with the seriously hairy document filter.

Associated stories: 
https://www.pivotaltracker.com/story/show/65469144
https://www.pivotaltracker.com/story/show/65361890