For get and filter queries of NamedUserGroup, realm_for_sharding
field is used instead of realm field, as directly using
realm_for_sharding field on NamedUserGroup makes the query faster
than using realm present on the base UserGroup table.
Anonymous groups were being created for stream permission
settings when calling the subscriptions endpoint without
any streams data or when calling it only for subscribing
users to streams and not for creating any new streams.
This commit makes sure that no such unused anonymous groups
are created.
