Fix several SMS bot issues, #133

 - Send instructions on first validation
 - Properly skip completed fields
 - Unify transcript images, and correct implementation of endpoint
 - Catch and refuse video MMS
 - Handle STOP, UNSUBSCRIBE, START, etc.
   https://www.twilio.com/help/faq/sms/does-twilio-support-stop-block-and-cancel-aka-sms-filtering