Use registries for review request fields and field sets

Review Request #7795 — Created Dec. 3, 2015 and submitted — Latest diff uploaded

Information

Review Board
release-2.6.x

Reviewers

Review request fields and fieldsets are now registered via registry
objects, however the external API has not changed.

A new Registry has been created that subclasses Djblet's Registry that
will never throw an ItemLookupError -- this is used becuase all Review
Board registries follow this pattern.

Ran unit tests.