Create API endpoint to trigger reminder emails

This is the start of functionality to send reminder emails. This PR should probably wait until the sms reminders branch is merged and then this can be rebased and finalized.

Todo
- [ ] Determine if "Trigger" is the appropriate resource name
- [ ] Fix code formatting (need https://github.com/codeforamerica/denver-schedules-api/issues/14 to be completed)
- [ ] Need real implementation of [FetchDueReminders](https://github.com/gregoryjscott/denver-schedules-api/blob/email-triggers/Schedules.API/Tasks/Reminders/FetchDueReminders.cs)
- [ ] Update documentation with Mandrill environment variables