Make activity group-specific

[mbramson]

Currently, users can be active or inactive globally for the purposes of pairing. Add the ability to toggle user activity for specific groups individually.

Users will want to be able to be active for certain group pairings, but not others.

This card should leave the is_active field on Users present, as using this group-specific activity is outside the scope of this card, as it will be a part of redesigning pairing.