Optimize loading of group leaders in /groups endpoiont

[ErikBoesen]

Under the API endpoint to get groups, we should make sure we're loading the leaders in the same request to speed up fetching. (It's not that slow right now, but always best to optimize.)

In Yale Menus, we do something like this to load a relationship at the same time:

        institution = Institution.query.options(joinedload(Institution.allergen_profile)).get(g.user.institution_id)