Treat first editions as 'major' changes

https://www.pivotaltracker.com/story/show/83478146

This changes the `PublishedEditionPresenter` so that, when presenting the first edition in a series, it always sets the `update_type` field to `major` change. 

Previously, first editions would never be sent as `major` updates, as the UI is not shown to users. However, we always want to send email alerts for first editions of content, and display it in the latest changes feed for a sub-topic.