require confirmation to DESTROY channel

Value

• DESTROY is a dangerous command that will take the non-reversible action of destroying an entire channel. (it is also a word it is not hard to imagine activists using in a message they didn't intend as a command.)
• it would be nice to give admins a chance to change their mind or correct a mistaken invocation by offering an "are you sure?" -> confirm flow when handling this command

Behavior

GIVEN an admin alice on a channel #foo

• WHEN alice issues DESTROY to #foo
• THEN alice receives a message that says "Are you sure you want to permanently destroy this channel and all records associated with it? Respond with CONFIRM DESTROY if so."
• IF alice responds with CONFIRM DESTROY then all records associated with the channel are deleted and she receives a message alerting her to this fact. (As per specs in #106 (closed))
• IF alice does not respond, then nothing happens