Add `revoke` functionality

[silverbucket]

Currently there's no way to revoke access for apps via. the API. Using the simple account dashboard, we should be able to list all apps that have access to our remote storage and optionally revoke their token with a simple click.
See #47

[DougReeder]

This is still highly desirable.

Implementing it would be straightforward for the monolithic server, as it stores permissions for each app.

The modular server uses JSON Web Tokens, so checking permissions doesn't require a round-trip to storage. Implementing revoke would require storing the app permissions and setting up a revocation list.