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/
Fix the image used for the New Review Request page in the release notes.

This was referencing the full screenshot and not the thumbnail. We're
now using the thumbnail, so that we can get the proper size and the
high-DPI version.