History and diff on commit approval page

[gavinwahl]

On the review queue commit approval page, It'd be nice to see the diff for the commit we're approving. Also maybe the history of the page.

• Should the diff be between the commit and its parent, or between the commit and the current version of the page?
• Pages can add multiple preview buttons (for multiple languages, for example). I guess we'd have to show the diff for each one? VersionTrackerMixin.get_diff_urls has this logic, but I'm not sure how to share that code with VersionCommitAdmin.