Enable retrieval of app usage event by guid

The `app_usage_events_controller` does not include the standard `define_routes` call to the base model controller since there's no need to provide all CRUD operations.  That's fine but the existing base controller will return enumerations with url's for individual events that were not handled by the `app_usage_events_controller`.

This simply adds the ability to get usage events by guid so the urls returned from the enumeration work and the behavior is consistent.