R1:90df4b2bd153

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…