When all actions in a submenu are disabled, disable the submenu header

Summary: Fixes T11240. Also simplify things a little and share a bit more code.

Test Plan:

• Viewed revisions and tasks, opened submenu.
• Viewed as a user without edit permission, saw the menus greyed out.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11240

Differential Revision: https://secure.phabricator.com/D16201