Add methods to Edition to return major updates

https://www.pivotaltracker.com/story/show/82952014

This changes the `Edition` model to include methods to return major updates in the edition series. It also includes methods to return the latest change note for the series, and the `updated_at` timestamp for the latest major update.

Some of this behaviour previously existed in the 'enhancements' of the `Edition` model in Publisher.
This use of = in an elsif clause looks very like a mistaken use of = instead of ==.  While I believe it is deliberate and correct, expanding this would make the code less surprising to read.