Added a summarizeForUser() method to the backpack connect model.

This allows us to easily get the union of all permissions for all tokens
issued to a user, grouped by origin, which will be useful for the
settings page, where a user can see what permissions each origin has, and
optionally revoke them.
added methods to backpack connect model, controller for revoking tokens.
First attempt at a working settings view for revoking issuer connections.
removed extraneous logging call
Added destroy() class method to mysql-base model.

This method is tested by the test for backpack-connect.revokeOriginForUser().
Made a new view/permissions.html partial. bp connect, settings now use it.
added integration tests for revoke-origin endpoint
renamed mysql-base.destroy to findAndDestroy().
removed dead code.
Merge pull request #664 from toolness/revoke-connections

Allow users to revoke connections previously set up by backpack connect