Allow stacked comment actions to be explicitly ordered

Summary:
Ref T6027. Normally, actions use the same order as the form, but in some cases (like moving stuff on workboards) it makes sense to reorder them explicitly.

Pin "Move on board" near the bottom, and "projects/subscribers" at the bottom. I think these are generally reasonable rules in all cases.

Test Plan: Opened menu, saw slightly better action order.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T6027

Differential Revision: https://secure.phabricator.com/D15639