Add public_timestamp to mainstream schema

Public timestamp exists already for whitehall indexes.  We want to be
able to have a comparable timestamp, holding the date of the most recent
major update to a page, for documents from publisher, too (so that we
can put them in the correct order in the 'latest' feed pages).  This
will be populated by future pull requests.
Merge pull request #311 from alphagov/add-public-timestamp-for-mainstream

Add public_timestamp to mainstream schema