R1:b1b1ff83f239

Allow applications to define new policy capabilities

Summary:
Ref T603. I want to let applications define new capabilities (like "can manage global rules" in Herald) and get full support for them, including reasonable error strings in the UI.

Currently, this is difficult for a couple of reasons. Partly this is just a code organization issue, which is easy to fix. The bigger thing is that we have a bunch of strings which depend on both the policy and capability, like: "You must be an administrator to view…