Reverting published_date changes

Our work regarding only setting published_at did not work as expected.
This commit reverts those changes in order for us to start with a clean slate
when setting the published_at date for SpecialistDocuments
Store public_updated_at for sorting finders and frontend updated_at

As the repository only pulls out the last 2 document editions there is no
garauntee that we will always have pulled out a document edition that is a
major update. Instead we will create a field which stores the date of publish
for the last major update. This is persisted across editions and exported to
rummager and content-api as the date which defines when a document was
last updated from the front end user's perpective.
Merge pull request #352 from alphagov/bug/published-dates

Published dates not updated on minor update