waitlist for pending channel requests

value

• after !528 (merged) if we let our channel reserve drop to 0 (which we might do intentionally to throttle user growth pending performance improvments), users who try to create a channel will simply have their request dropped and be told to try again later
• but we would still like these people to be able to get channels later once we have capacity without having to "try again!"
• this issue introduces a "wait list" that users will be added to that will allow maintainers to go back and manually create channels for them once capacity frees up

behavior

GIVEN that signalboost's stock of verified phone numbers has dropped to 0

• WHEN a user submits a valid CHANNEL command
• THEN they will receive a message telling them they have been added to a waitlist and will be notified when their channel has been created
• AND THEN the admin phone numbers will be stored in a channelRequests table (in a JSON array)

out of scope

• managing the waitlist is MANUAL for now. we leave automation of taking people OFF the waitlist to #455