Added sandboxing to ReviewRequestFieldset

This change adds sandboxing for the various ways that extensions can add
fieldsets and fields to review requests. This also adds unit tests.

Testing Done:
Unit testing for init function for BaseReviewRequestFieldSet and
BaseReviewRequestField, is_empty for BaseReviewRequestFieldSet, and
should_render for BaseReviewRequestField

Reviewed at https://reviews.reviewboard.org/r/5738/
Merge branch 'release-2.0.x'