Allow comment actions to be grouped; group Differential "Review" and "Revision" actions

Summary:
Ref T11114. Differential has more actions than it once did, and may have further actions in the future.

Make this dropdown a little easier to parse by grouping similar types of actions, like "Accept" and "Reject".

(The action order still needs to be tweaked a bit.)

Test Plan:

Reviewers: chad

Reviewed By: chad

Subscribers: eadler

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17114