R1:d0d49d1efdaf

Allow Portals to be edited, and improve empty/blank states

Summary:
Depends on D20348. Ref T13275. Portals are mostly just a "ProfileMenuEngine" menu, and that code is already relatively modular/flexible, so set that up to start with.

The stuff it gets wrong right now is mostly around empty/no-permission states, since the original use cases (project menus) didn't have any of these states: it's not possible to have a project menu with no content.

Let the engine render an "empty" state (when there are no…