Group Membership Validity Dates

Attach validity dates to CO Group Memberships.

This implies cleanup to only ever have one row per CO Person/CO Group. Implement CoGroup::setMember(), setOwner(), removeMember(), removeOwner()? or similar to improve handling.

Initially no need for an owner and member to expire separately, and maybe that shouldn't be a native use case. (ie: Use Grouper instead.)

Status on group membership may be required so expirations can check to see if the membership was already removed (and therefore need not be reprovisioned).

Set group expiration date as part of enrollment flow?

Group owners can edit membership validity dates (unless linked to OIS).