[#397] Automate channel creation

context

• in order to limit the flow of new channel creation (and thus overloading our servers if that flow grew too large), we currently process channel creation requests manually from messages submitted to BOOST IT
• although these messages disappear, it leaks information to us that we would rather not know!
• it is also time consuming and we have lots of other stuff to do :)

changes

pls see spec bc there are hella corner cases! #379 (closed)

execute layer

• new REQUEST command
• new CHANNEL command that calls registrar#create

registrar layer

• alteration of the signature of registrar#create to take admin phone numbers and optionally a cli-specified phone number (this arg could use a better name?)
• new channelCreationAttempt (also could prob use a better name) notification to maintainers

questions for code review

some things came up in QA just now

• do we want to enforce 2 admins/channel? right now it does not. this seems like a good idea but unsure how.
• in testing happy path as a rando I discovered what I think is a bug in the invite-to-support-channel logic.
  • in channelRegistrar#_inviteToSupportChannel line #127 (closed) we send a welcome notification from the support channel, which says ``You were just made an admin of this Signalboost channel by ${addingAdmin}. Welcome!`
  • this doesn't make sense? I think we mean to send something like You have been invited to this channel...
  • however, now that we have channel requests going thru BOOSTIT I feel like it's overwhelming to get a (1) support channel invite (2) command response saying channel has been created and (3) welcome notification from the new channel all at once. I could do away with the support channel invite (or add something to the REQUEST cr?) but curious as to your thoughts.

Closes #379 (closed) (oops - @aguestuser I numbered this branch incorrectly (379 not 397), lmk if you want to change that!)