Add /abandon command usable by application thread founder

[ShadyMedic]

This would be a "soft" variant to /close immediately, that the voice actors could use to abandon the application thread.

Running /abandon in the application thread would cause:

1. Checking that the command sender is the voice actor who founded that thread (probably by checking its username matches the second part of the thread's name)
2. Removing/kicking the command sender from the private application thread
3. Pinging @CastManager to check the thread for anything worth keeping or deleting it with /close immediately.

This would let non-staff voice actors to opt-out of the thread immediately without needing to wait for a staff member, while keeping the thread safe for any desired checks. If the Cast Managers decide that the thread has an audition that is good, they might try to convince the voice actor to not give up on the audition and re-add him back to the private thread by pinging them in it.