Make Institution Group single-valued

It seems that by removing "Public" as an Institution Group, we can use the existing Institution Group relation to satisfy our needs for identifying an institution's Type unambiguously. This is an essential component to supporting a Wayfinder and customized kids user views.

If this is OK, we need to:

1. Clean up DOOR data to ensure no Institutions are in more than 1 group BESIDES public. Problem records include 'special' institutions, UGA, perhaps others. Also remove inactive institution group.
2. Change the relation from has_many to has_one
3. Ignore public on ingest ONLY when getting Institution Group
4. Have alternate solution for EBSCO (and other) Test sites (e.g., remove public allocations)