Break a circular reference in diffutils.

diffutils imports from Profile, which eventually imports diffutils
again, through a chain of imports. Depending on what's been imported
first, things are fine, and in fact, Review Board itself has no problems
with this. However, an extension trying to import from diffutils will
end up hitting this issue.

We only had the Profile import for the purpose of the DoesNotExist
exception. Instead, we can easily replace that with the more generic
ObjectDoesNotExist, breaking this cycle.

Reviewed at https://reviews.reviewboard.org/r/5839/
Fix an issue saving file attachments after guessing the mimetype.

In order to guess the mimetype for a file attachment, we'd feed the
contents to `file`. This worked fine, except that we never seeked the
position back to 0, causing issues when trying to write the file. This
really upset boto, breaking S3 support for any files we didn't have a
mimetype for.

The fix is simple. Just seek back to 0.

Reviewed at https://reviews.reviewboard.org/r/5842/
Add the changes made since 2.0 RC 3 to the release notes.

Our final releases always had a list of changes since the last RC
release, but we didn't have that in these release notes. This adds that
section with all the latest commits (that matter).

Reviewed at https://reviews.reviewboard.org/r/5843/
Release Review Board 2.0.
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.
Remove the "in-development" from the 2.0 Releases section.
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.
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/