Revisions: Pinned controls, layout tweaks, copy tweaks, misc.

* When you scroll down the diff view, the controls will pin to the top.
* The revisions meta view was cleaned up. Copy changes.
* Loading indicator in the center of the screen (so it follows as you scroll).
* Tooltips "flip" when you cross the center line, so that they don't hit the container edge and wrap for later revisions.
* The "Restore" button's inactive state is handled on render, instead of after.
* Make sure we always have a current revision, even if the timestamp doesn't work out on the most recent one.

See #24804. Props markjaquith, nacin, ocean90, aaroncampbell.

git-svn-id: http://core.svn.wordpress.org/trunk@24761 1a063a9b-81f0-0310-95a4-ce76da25c4cd