CRM-13244 - CRM_Core_BAO_ActionSchedule - Simplify `sendReminder`

CRM-13244 - CRM_Core_BAO_ActionSchedule - Simplify `sendReminder`

The old `sendReminder()` had too many responsibilities (loading token data
and processing token data and sending emails and sending SMS's), contained
lots of duplicate code, and did pointless variable shuffling.  The Smarty
and HTML parameters were enforced inconsistently, and the "From" address had
weird side-effects, ad nauseum.

This commit splits `sendReminder()` into three pieces:
 * `TokenCompatSubscriber` for filling in token data.
 * `sendReminderSms()` for sending SMS.
 * `sendReminderEmail()` for sending email.