Support multiple languages (with Spanish to start)

This PR implements:

- Spanish language support for CA and MO
- A major refactor enabling easy addition of new languages
- Language configuration via Twilio phone number 'friendly names'
  - Example: to set up a Spanish phone line, configure a Twilio number to have 'spanish' in its friendly name (e.g., `massachusetts-spanish` or `la-food-bank-spanish`)
- To support a new language, there are a few steps involved — @daguar should handle it manually for now

Needs to be fully tested on a staging server, but we're all green, baby!