Scheduled Reminders Integration

The goal is to give Volunteer Projects feature parity with CiviEvents as far as Scheduled Reminders are concerned.

Currently, users can send reminders to Volunteers by selecting Activity as the entity and Volunteer as the activity type. With a few new tokens (see VOL-112), we'll be able to send messages with reasonably good context about the Project, but users won't have the ability the filter the recipients; in other words, though the messages will have Project-specific context, the pool of recipients will always be all volunteers, and the schedule of messages will be global.

Presenting Volunteer Project as a first-class entity to Scheduled Reminders would allow us to filter participants by Project and/or Volunteer Role. For example, I could create a scheduled reminder that targets only volunteers with the role of Lifeguard in the volunteer project High School Swim Meet. This kind of filtering is already possible with Events.

Unfortunately, extending Scheduled Reminders appears to be a somewhat unblazed trail. All the enhancements to this feature have gone directly into core; there are no classes to extend or hooks to implement if you wish to change the existing behavior via an extension. The features we want will require changes to core. It's quite possible these changes will be too big to go into a 4.6.x release.

Questions/challenges not specifically related to core/extensibility:
=================================================

• Volunteer Projects don't currently have start times. In 1.4-land, we can get this information from the parent Event. In 2.0-land, Projects may not be associated with Events... should Projects have an optional start time? Should reminders be based off of Needs or Activities rather than Projects? Should the user be able to choose?