Bug/published dates

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 all documents that have been published should have
a PublicationLog entry from this point on. For any that do not have a 
PublicationLog entry just fall back to the documents updated_at as before.

There is some discussion about pulling more documents from the repository
however we are worried about taking a huge hit on performance which is already an issue here.

Without a migration to create a PublicationLog for each document that doesn't currently have one, documents that are already published will continue to have their last_update touched each time they're edited, until such time as they have a major edit performed on them.