add signup channels

specs:

• given an env var specified SIGNUP_CHANNEL_NUMBER
• when a person sends a signal message to that number
• then admins of the channel (who must be signalboost maintainers) will receive a disappearing message containing that person's number (so they can create a channel for the person
• the message sender will receive a response letting them know they will get a welcome message from their new channel's number once it is creeated