Removing all channels

[cespare]

You can remove all channels right now, and it leaves you in kind of a weird state. (You can type in messages, but they don't get sent anywhere. Also loading the page when you've left all channels leaves you in the 'loading' state indefinitely.)

We should probably prevent this. The fix could be as simple as not allowing the user to delete the channel (even remove the trashcan) if it's the only one remaining.