[92] suggest help more helpfully

behavior changes:

• when a user first joins, give them a short message that:
  • suggests they ask for HELP
  • tells them how to LEAVE
• move the context of how signalboost works out of welcome message and into INFO response
• when a user sends an unauthorized broadcast, respond by suggesting they send HELP

side-effects:

• extract prefixes into messages file (for easier localization)

Closes #92 (closed)