[#105] Resolve "respond to sms messages"

Closes #105 (closed)

Context:

• we would like to provide fallback SMS messages to new users to try to send "HELLO" (or the like) to signalboost numbers b/c they don't understand or yet have signal
• but we also don't want to open signalboost up to a resource starvation attack from a malicious user who floods us with text messages we can't pay to respond to
• this MR provides a method for responding to SMS messages only for users who have not exceeded a quota of 3 inbound messages (which is reset every month)

Changes

Foundation

• add a smsSenders table

  • use it to track who sent incoming messages and when

• add an smsSenderRepository

  • #countMessage increments the messagesSent count of an smsSender
  • #hasReachedQuota checks whether a sender has reached their quota of responses (currently 3/month)
  • #deleteExpired deletes all records older than a month (to preserve privacy of sms senders and allow quotas to reset)

• add language module

  • extract constants for supported languages from app.constants to here
  • #languageForPhoneNumber leverages new libphonenumber-js dependency to parse the country code from a phone number, and use that to infer the (most likely) language spoken by the sms sender (via country -> language mapping provided inlanguageForCountry)

Interface

• to handle incoming messages:

  • add registrar.sms module, use it to expose #handleSms to the /twilioSms route, which:
    • tries to parse a signal verification code from a message, and if successful, dispatches to verification flow in registrar.register.verify
    • if message is not a verification code, it:
      • use smsRepository.hasReachedQuota to see if we should respond
      • if so, use languages.languageForPhoneNumber to generate localized response to wrap in a TWIML message (where TWIML is twilio-flavored XML -- a thing we can respond to twilio callbacks with to get it to send sms's with arbitrary content)

• for record scrubbing:

  • call `` from the main loop in registrar.run
  • since this runs roughly every 24 hours after db database backup restarts the app, we have a loose guarantee that sms sender records older than 1 month will be deleted every day