user_groups: Use atomic transaction for adding/removing members.
user_groups: Use atomic transaction for adding/removing members.

We execute code for bulk adding and removing members from a user
group inside a transaction.

rht/zulip,rht/zulip,kou/zulip,kou/zulip,kou/zulip,kou/zulip,zulip/zulip,rht/zulip,andersk/zulip,andersk/zulip,zulip/zulip,andersk/zulip,rht/zulip,zulip/zulip,kou/zulip,andersk/zulip,zulip/zulip,andersk/zulip,zulip/zulip,rht/zulip,kou/zulip,andersk/zulip,rht/zulip,rht/zulip,kou/zulip,andersk/zulip,zulip/zulip,zulip/zulip