Add a new revision selector UI.

This change gets rid of our old rows of boxes in favor of a new slider widget.
The slider has ticks for "orig" (the original revision) and each diff revision.
By sliding one of the two handles around, the user can select any individual
diff revision, or an interdiff.

There's currently one shortcut, where you can click an individual number to
jump straight to that diff. I've added a pointer cursor here to help make this
discoverable. I'd also like to add the ability to click and drag across two
labels to jump to an interdiff.

The rest of the UI around this (the revision label, comments hint, and others)
all jump around way more than I'd like. I'm currently playing with a few
designs to make it so nothing will move around when the info changes.

Testing done:
- Played around a lot with the mouse handling trying to get it to break. The
  "grab" works pretty well.
- Selected various revisions and interdiff revisions. Saw that the correct one
  was loaded all the time.
- Checked that the UI didn't show up when viewing a single revision.

Reviewed at https://reviews.reviewboard.org/r/4695/