Send information about an edition's latest change to Panopticon

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https://www.pivotaltracker.com/story/show/82952014

This changes Publisher so that the timestamp of the most recent major change to an edition, along with its change note, are included when we register an edition with Panopticon. 

An [existing change to Panopticon](https://github.com/alphagov/panopticon/pull/234) means that these fields will be indexed in Rummager, which in turn means that the change notes will appear to users in the latest changes feed for a sub-topic.

This also changes the presenter which renders editions for the Publishing API so that we now send the change note for the most recent _major_ change, rather than the change note for whichever edition was published most recently (which could be a minor change).