add rake gitlab:import: all_users_to_all_groups and user_to_groups

I opted for admins to be added as "owners" instead of "masters" because project masters can managers members, but only group owners can manage members.
Merge pull request #5520 from gabetax/rake_group_bulk_add_permissions

add rake gitlab:import: all_users_to_all_groups and user_to_groups