Feature Request: Custom auto-responses for different keywords

[crayolakat]

Problem
After switching from Twilio to Bandwidth, we lost Twilio's built-in feature to auto-send a response for keywords. In particular, HELP and STOP. We can use @agreenspan24's auto-optout response feature (https://github.com/MoveOnOrg/Spoke/pull/2010) to automatically send the organization's default opt-out message when someone texts STOP, but not for the HELP keyword.

Solution
Build an auto-respond message handler to sent a custom message for various keywords, such as HELP

[crayolakat]

It would also be helpful to automatically opt people back in if they text a keyword. For example, here are Twilio's default responses for the STOP and START keywords:

• STOP: You have successfully been unsubscribed. You will not receive any more messages from this number. Reply START to resubscribe.
• START: You have successfully been re-subscribed to messages from this number. Reply HELP for help. Reply STOP to unsubscribe. Msg&Data Rates May Apply.