[#199] Resolve "HOTFIX: respond gracefully to bad JOIN attempts"

Closes #199 (closed)

Context

• People send all sorts of odd things instead of HELLO. Variants include Hello!, Hey! etc. We want to make it easier for these people to join without making them do too much work.

Changes

• Accept trailing punctuation marks after a HELLO command (along with more slang variants on "hello")
• Explicitly mention the HELLO command in the notification sent in response to an incoming hotline message (give it greater emphasis by mentioning it before the "include your number" stuff)