Fix publication dating (55389442)

This resolves the issue where publications were inadvertently being backdated when they were published after the day they were initially draft. This is because Publications had their own special "publication_date", which was being set at the point the edition was first saved. This code drops the use of this column and instead has publications working just like other editions, i.e. they have a "first_published_at" attribute which, if left blank, is automatically set when the edition is first published. This can still be manually set for backdated publications both in the main editing view and the importer.

I've also dropped an old rake task that we don't need anymore, rather than update it to use the new column.

Tracker: https://www.pivotaltracker.com/story/show/55389442