R1:d2572f8b335a

Refine more Differential review state behaviors

Summary:
Ref T13516.

- Add an "Add Comment" navigation anchor.
- Make selection state more clear.
- Make hidden state tidier and more clear.
- Hide "View Options" in the hidden state to dodge all the weird behaviors it implies.
- Click to select/deselect changesets.
- When you open the view dropdown menu, then press "h", close the dropdown menu.

Test Plan: Fiddled with all these behaviors.

Maniphest Tasks: T13516

Differential Revision: https://secure.