list_groups service: evaluate eager-loading SQLA relations

(Performance-related)

As I understand it, by default, SQLA will lazy-load model relations when you reference them. That's less optimal if you have a list of models and end up accessing relations on each of them—that's what we're doing when we built `GroupResource` objects; each `Group` has an expanded `organization` on it.

Perhaps change the query/ies in this service to eager-load objects by default in the `list-groups` service as we know we'll be expanding the returned Groups in many cases.