invalidate user's cached groups on membership update

fixes CNVS-5256

test plan
- as a teacher, add a user to a group
- wait 15 minutes or clear the "Courses & Groups" cache entry
- as the user, ensure that the new group shows up in the "Courses
 & Groups" menu
- as the teacher, remove the user from the group
- wait 15 minutes or clear the "Courses & Groups" cache entry
- as the user, ensure that the new group does not show up in the
 "Courses & Groups" menu

Change-Id: I091775f50783e0f9bb99a4adbc3e726a7f78be89
Reviewed-on: https://gerrit.instructure.com/45603
Reviewed-by: Andrew Butterfield <abutterfield@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Derek Hansen <dhansen@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>