Separate Fedora and Hydra rels-ext terms

This moves the Fedora rels-ext terms to a new class with a new url that resolves to their new location. Hydra rels-ext terms are given their own class for clarity.

We should explore opportunities for parity with #616 . @barmintor suggested keeping class names the same in both AF8 and AF9 releases which would make migrating less painful.
How much of this test is testing the private API vs. the public? I'm wondering if the test could, or should, be rejiggered to rely more on the public side if it doesn't already.