Add Permissions Model.

This feature is a WIP.

Future PRs:
1) Add UI for App owners to specify permissions and provide descriptions.
2) Update Core to use permissions, including having a way to populate the perms DB with core permissions.
3) Add UI for viewing permissions, including users and groups that have those permissions.
4) Add UI for assigning permissions to groups and users.
Hrm. I know this is a pattern we use elsewhere, I don't like it very much though. Importing at the top level means that if there is a problem we discover it early, instead of whenever we first execute this code path.