Refactor the FieldSetRegistry to use Djblets' OrderedRegistry

Review Request #7835 — Created Dec. 28, 2015 and discarded — Latest diff uploaded

Information

Review Board

Reviewers

The `FieldSetRegistry` now takes advantage of Djblets' built-in
`OrderedRegistry` class.

Testing Done:
- Ran unit tests.

Reviewed at https://reviews.reviewboard.org/r/7816/

a test