[Placeholder] Frontend part of announceable statistics

-[ ] Add front-end model / struct for ReleaseAnnouncements
-[ ] Stub rummager response
-[ ] Add "Upcoming calendar" tab to statistics index page showing all release announcements
-[ ] Spike into whether existing filters can be nicely persuaded to handle non-document rummager content
-[ ] Add filter form, either reusing existing filter or creating a new one.  Task finishes with rummager stub receiving filter params and the results it responds with being rendered to the page.
-[ ] Normalise front-end release announcements schema with back-end solution and remove the rummager stub.

Associated stories: 
https://www.pivotaltracker.com/story/show/65469144
https://www.pivotaltracker.com/story/show/65361890