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 items that can render a content page) and try to make some of the empty behavior a little more user-friendly.

This mostly makes portals work, more or less.

Test Plan: {F6322284}

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13275

Differential Revision: https://secure.phabricator.com/D20349