Move group API active user precompute mechanics into a Group subclass
Move group API active user precompute mechanics into a Group subclass

This also fixes a correctness issue outlined here:
https://github.com/dimagi/commcare-hq/pull/25532#discussion_r331132430

dimagi/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq