Remove magic strings / references to Duke

This program interfaces with a few pieces of infrastructure at Duke: OIDC for OAuth, Grouper for group membership, and Duke Data Service for data.

OIDC and grouper are not Duke-specific, so the code should reflect that it works with these projects/standards (e.g. change the specific dukeUniqueID fields to something like accountID if valid).

Duke Data Service, on the other hand, is the name of that singular project, so it follows that identifiers here relating to that project would follow its name.