Clean ups and locks

Delete a batch of messages, lock a channel or slow it down.

• /purge: Delete a batch of recent messages, for staff
• /lock: Stop everyone from sending messages in a channel, for staff
• /unlock: Let everyone send messages in a channel again, for staff
• /slowmode: Set a channel's slowmode, for staff
• /snipe: Show the last deleted or edited message in this channel, for staff