[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)