Resolve #103: "tech task: cleanup signup channels"

Closes #103 (closed)

• add unit tests for signup channel message handling
• block non-admin command execution on signup channel
  • this ensures that nobody can ever subscribe, so admin messages will never be broadcast
  • that means admins can also use signup channel to talk to each other about fielding signups
  • it also means no signup messages will fail to result in signups because they are mistaken for commands
• tweaks:
  • rename config signupChannel -> signupPhoneNumber
  • change dev signup channel