An easy way to turn on an auto-responder (directing users to call phone # instead)

[daguar]

We've turned the system off manually a few times (most recently #286) by setting up static responses that, per state, direct any user who texts in to call the state phone line instead (which we have in the code base.)

In anticipation of future outages (a state line goes down, Twilio has problems) it would be useful to have a switch we could flip to turn on this fallback response.

First thought on a simple implementation:

• An ENV var (AUTO_RESPONDER_ON) would be how this would be turned on
• When on, the user receives a friendly text telling them to check back soon, but that they can check their balance now by calling the state line (in the code base)

Maybe improvements:

• Allow doing this for specific states only