Admin endpoint to reject timeslot requests.

[BinaryFissionGames]

We need an endpoint to reject timeslot requests. This will, like other endpoints, take parameters through the url (see documentation about it here), then will delete the timeslot request from the database.

The user who made the timeslot should be notified of the rejection.

[BinaryFissionGames]

Done, but notification of rejection probably should be more specific.