HOTFIX: respond gracefully to bad JOIN attempts

This is a messily-specc'ed card b/c it is an fast-turnaround hotfix in response to a user request. Sorry!

Value

• 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.
• Do that with more relaxed parsing rules for HELLO and by explicitly mentioning the HELLO command in the notification sent in response to an incoming hotline message

Behavior

GIVEN a JOIN command that includes punctuation after hello

• THEN it will be interpretted as HELLO

GIVEN an unsubscribed user on a channel with hotline messages on

• WHEN the user sends a message to the channel
• THEN the notification they receive in response will mention HELLO in the second sentence