Cleanup of the revisions screen, both on the PHP API side, and the JS.

* Much simpler PHP API
* Cleaner and more Backbone-y JS API
* Consequently, does batch queries; this now scales up to hundreds of revisions

Currently missing, but much easier considering the cleaned up base:

* Compare two mode
* RTL

props koopersmith, nacin, adamsilverstein, ocean90. see #24425

git-svn-id: https://core.svn.wordpress.org/trunk@24520 1a063a9b-81f0-0310-95a4-ce76da25c4cd