Added new stats release announcement fields

A new content type is being added to whitehall called `StatisticalReleaseAnnouncement`. This will be used to pre-announce forthcoming statistical releases. We require two extra fields in the search index to allow us to list these in search results:

  1. `expected_release_timestamp` - a timestamp representing the expected release of the stats for sorting purposes (so we can show the soonest first) 
  2. `expected_release_text` - a textual version of the expected release date to be displayed on the site, as the timestamp will sometimes only be an approximation, e.g. "April 2015".