Add API for getting change descriptions.

This introduces new API for getting change descriptions on a review request.
It will return all public change descriptions, along with the text, timestamp,
and a list of fields changed. The list of fields changed is a more detailed,
useful list than what is stored internally. We expand it out, embedding
resources with links and providing a useful representation for the data.

Unit tests pass.

Verified through the browser that I got data that made sense. Tested with
every combination of changes.