Remove deleted agents from supervisor

[bracesproul]

If you create a supervisor agent, attach some agents to it, then delete one/many of the sub agents, there is no way to remove them from the supervisor right now.

We should make it so whenever you load the edit supervisor form, and we add the existing agents to the sub-agent select, we auto-remove any agents which no longer exist. then, if the user saves, those changes will be sent to the server.

The one downside of this, is if you delete a sub-agent, it's not automatically removed from any supervisors it's attached to, until the user "saves" the supervisor. I think nbd right now, but something to think about & document