Add a published_at field to RenderedSpecialistDocument

To avoid updated_at being modified every time we create a new draft, we
want a new 'published_at' field which is set only upon publish.
Update republishing script to work for aaib reports and IDF's
Merge pull request #199 from alphagov/set-published-at-on-rsds

Add a published_at field to RenderedSpecialistDocument