Multiple membership renewal reminders

Administrators will be able to configure an unlimited number of reminders at specified dates relative to the membership expiration, and configure different message templates as needed for each reminder. We will leverage the Scheduled Reminders framework for this.

--------- Detailed requirements from Jay McGraw (lead sponsor) ---------

Schedule renewal reminders using different message templates per reminder up to and after a membership expires. For instance, a standard process for us is to send a renewal reminder 30 days before the membership expires, another (different message) 15 days in advance, and another (different message) 15 days after the membership expires. In all these cases, further reminders would NOT be sent if the membership was renewed.

Set different schedules/messages per membership type.

Allow for some filtering on who receives these reminders. At minimum, we should be able to filter by Membership Type, Status, Group, similar to what you've documented for "Scheduled Reminders for Events".

Membership tokens and contact tokens available in those reminders, at minimum. Need to be able to say something like, "Dear [first name], your [membership type] is expiring on [end date]." Having the membership fee (dues) for that membership type available as a token would be useful as well.

Ability to have a link to online renewal form (contribution page) with checksum tokens so form autopopulated for member.

Anytime a reminder is sent, an activity is created. Something like "Membership Reminder Sent". It would be nice, but not necessary if it indicated which reminder was sent and/or contained the message text in the activity description.