Add Retina versions of the 2.0 release note screenshots.

These were missing before, but we had copies from the user manual, so
now we're just including those.
Merge branch 'release-2.0.x'
Remove the "in-development" from the 2.0 Releases section.
Merge branch 'release-2.0.x'
Remove the clickability of the New Review Request thumbnail.

The New Review Request thumbnail in the release notes was clickable, but
the website wasn't serving up the main image, showing a 404 instead.

Since the thumbnail has been made big enough to read pretty well, I've
just removed the ability to click it.
Merge branch 'release-2.0.x'
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'