Rename update timestamp to public

With the work to create updates from publication logs we mistakingly
renamed this. This commit corrects it to public_timestamp like other
document formats.
Explain major and minor for publishing

As Users now have change notes, we should probably explain what type of
change it is and what impact publishing will have.

Ticket:
https://trello.com/c/mUPdaenr/426-minor-major-edits-confirmation-on-publish-2
Merge pull request #349 from alphagov/rename-update-timestamp-to-public

Rename update timestamp to public
Merge pull request #350 from alphagov/major-minor-explanation

Explain major and minor for publishing
Fix Publication Log Change History migration

Previous version of this migration returned SpecialistDocumentEditions
with empty array change notes history. This commit fixes the query to
only return editions with a change note history with something in it and
also adds a condition to make sure that the object pulled from the array
isn't nil.
Merge pull request #353 from alphagov/fix-publication-log-migration

Fix Publication Log Change History migration