Add skeleton for Portals, a collection of dashboards and other resources

Summary:
Ref T13275. Today, you can build a custom page on the home page, on project pages, and in your favorites menu.

PHI374 would approximately like to build a completely standalone custom page, and this generally seems like a reasonable capability which we should support, and which should be easy to support if the "custom menu" stuff is built right.

In the near future, I'm planning to shore up some of the outstanding issues with profile menus and then build charts (which will have a big dashboard/panel component), so adding Portals now should let me double up on a lot of the testing and maybe make some of it a bit easier.

Test Plan:
Viewed the list of portals, created a new portal. Everything is currently a pure skeleton with no unique behavior.

Here's a glorious portal page:

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13275

Differential Revision: https://secure.phabricator.com/D20348