Check for First Published as change note

Documents created in Specialist Publisher after change notes have been
added have "First published." as a change note instead of a blank
string. This commit changes the check to check for both states as older,
scraped content, still has the blank change note.

There's probably more work to refactor this out like a view adapter but
I will do that in another commit as this fix should be out sooner rather
than later.
Merge pull request #363 from alphagov/publish-form-drafts-message

Check for First Published as change note