Move Favorites and User menus to MenuBarExtensions

Summary:
Ref T12140. The major effect of this change is that uninstalling "Home" (as we do on admin.phacility.com) no longer uninstalls the user menu (which is required to access settings or log out).

This also simplifies the code a bit, by consolidating how menus are built into MenuBarExtensions instead of some in Applications and some in Extensions.

Test Plan:

• While logged in and logged out, saw main menus in the correct order.
• Uninstalled Favorites, saw the menu vanish.
• Uninstalled Home, still had a user menu.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12140

Differential Revision: https://secure.phabricator.com/D17239