Ask and verify guardian and mentor phone number (via twilio)

[karbassi]

Goal

We want to send mentors and guardians automated SMS messages when they sign up for a class and the day before the class. We need them to give us their phone number and verify it with twilio so we can send them messages.

Task:

• [ ] Have mentor and guardian DB models have a US-based phone number.
• [ ] Have Twilio send a test SMS to verify the phone number is real
• [ ] Send an SMS message when they register for a class.
  • [ ] The SMS should be delayed so if a guardian signs up multiple kids, they don't get multiple SMS messages, only one per account.
• [ ] Send an SMS message 24 hours before the class reminding them time and location. Make sure the SMS is sent between 10 am and 8 pm only.
• [ ] Send an SMS message 2 hours after class thanking them for coming.

Nice to have:

• [ ] Option for the mentor and guardian models to register multiple phone numbers with labels (such as "Mother", "Father") and have a checkbox if they should get SMS messages there.

Resources:

• https://www.twilio.com/docs/sms
• https://www.twilio.com/docs/sms/tutorials/appointment-reminders-python-django

Related to #749