Don't try to activate disabled items in dropdown menus

Summary: Fixes an issue where the user could click a disabled dropdown menu item and get an exception or some other nonsense. Instead, just don't activate anything.

Test Plan: Clicked a disabled header, like "Members of project" in the policy dropdown.

Reviewers: btrahan, va.multimoney, chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D7293