Store default Spotlights on InstitutionGroup

Now that we have cleaned up the InstitutionGroup configurations, here's how I see the "default Spotlight configs" issue sorting out.

We've discusses storing defaults on the InstitutionGroup, and now that the Institution relation is single-valued, this should be possible.

The SpotlightConfig can be turned into a polymorphic relation, allowing us to save SpotlightConfig records on the InstitutionGroup - following the same add_spotlight_configs logic as we do with institutions after_create.

We'll then be able to do this in our forms and indexing blocks:

institution = Institution.find_by(code: 'ath1') # ACC Libs
institution.institution_group.spotlight_configs.first # returns 'default' config with 'default' spotlight resources/links
institution.spotlight_configs.first # returns ACC Libraries custom customizations