Update IG Gitlab sync to remove members from groups

As stated in #213 (closed), we should remove users from groups that are marked as "terminated". This will allow us to effectively archive these groups once the interest group is no longer active. This is low priority as this won't impact us for the first year of operation of these new groups.

Setting due date to end of 2022 as a general goal for implementation, but we could easily push this back further if we get busy.