Timestamps

`public_timestamp`

point in time the edition became visible to the public on the website. Updated for major changes
used to sort documents in the atom feed
for building change_history on a document
used when comparing edition published dates in scopes on Edition (e.g. published_before(date))
set to first_public_at or major_change_published_at on every save

signifies when the document was 'first published', which may be before the public timestamp. E.g. transitioned content, etc.
Either user supplied on the form, or set during publishing to the major_change_published_at timestamp

date of the last major change. Major changes require change notes. This is decided by the user.