[IMP] base: compute users `share` in batch.

Instead of triggering one has_group by user (one sql query/user if not already ormcached),
and potentially filling the `has_group` cache with new users data (we don't know if and when this data
will be used anyway), compute this field from the groups information already in cache.

X-original-commit: d771d7e9c8503543d29fdbf2ab961c2a32cc3bac