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.phabricator.com/D21161