Merge "differential-dropdown-menus" behavior into DiffChangesetList

Summary: Ref T12616. This ends up being a little messy ("one giant function") and maybe I'll clean it up a bit later, but continue consolidating the wild jungle of behaviors into a smaller set of responsible objects.

Test Plan: Clicked all the menu options, saw them work properly. Grepped for removed methods.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12616

Differential Revision: https://secure.phabricator.com/D17845