Add text reminders

* Uses twilio for text reminders
* Loops through each reminder and sends to Twilio. Twilio queues the messages and sends them at a rate of one message per phone number per second. This rate is not adjustable unless an additional phone number is added to the plan.

TODO
* [x] Get due reminders and send them
* [x] Create send sms endpoint
* [x] Refactor to reuse code for email and sms
* [ ] Clean up code