Begin modularizing main menu items

Summary:
Ref T10077. Ref T8918. The way the main menu is built is not very modular and fairly hacky.

It assumes menus are provided by applications, but this isn't exactly true. Notably, the "Quick Create" menu is not per-application.

The current method of building this menu is very inefficient (see T10077). Particularly, we have to build it twice because we need to build it once to render the item and then again to render the dropdown options.

Start cleaning this up. This diff doesn't actually have any behavioral changes, since I can't swap the menu over until we get rid of all the other items and I haven't extended this to Notifications/Conpherence yet so it doesn't actually fix T8918.

Test Plan: Viewed menus while logged in, logged out, in different applications, in desktop/mobile. Nothing appeared different.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8918, T10077

Differential Revision: https://secure.phabricator.com/D14922