Allow users to revoke connections previously set up by backpack connect

This pull request isn't finished yet. Still need to:

- [x] Modify the settings view to list all connected origins and their permissions, with a "revoke" button next to each origin.
- [x] Add a controller that allows all tokens for a given domain to be revoked, which the aforementioned "revoke" button is connected to.
- [x] Get rid of an [extraneous logging statement](https://github.com/mozilla/openbadges/pull/664#discussion_r3246841).
- [x] Add a `destroy` class method to the `mysql-base` model and have `Session.revokeOriginForUser()` use it.
- [x] Move permission list into a partial that's shared with the `backpack-connect.html` view.