R1:971a272bf679

Automatically build mobile menus from navigation, and clean up external ProfileMenu API

Summary:
Depends on D20355. Ref T13275. Ref T13247. Currently, "Hamburger" menus are not automatically built from navigation menus. However, this is (I'm almost completely sure?) a reasonable and appropriate default behavior, and saves us some code around profile menus.

With this rule in place, we can remove `setApplicationMenu()` and `getApplicationMenu()` from `StandardPageView`, since they have no callers.

This…