Add the beginnings of review UI support.

This introduces the basic support for building review UIs. This allows
Review Board and extensions to register special UIs for reviewing
certain file attachments. An example would be a special UI for reviewing
XML files, or PDF files.

There's a base ReviewUI class that forms the base for anything that
provides review functionality for something else. It knows how to do the
basic rendering and grab the list of current comments. In time, we may
even want to move the diff viewer to this.

Then there's FileAttachmentReviewUI, which is where the magic will
really happen for us. It looks at registered handlers and tries to find
one that best matches the mimetype we're looking for. This is the same
logic as thumbnail renderer matching. (And we may want to consolidate
that logic in time.)

Review UIs are currently displayed in their own page, but they will
later be able to specify that they can review inline. In this case,
we'll just pop up a lightbox and load in the HTML for the review UI.

Right now, there's only one review UI, and that's for screenshots. This
is not a FileAttachmentReviewUI. I have one in-progress for image file
attachments. However, scanning for review UIs doesn't actually work in
practice very well, because it requires that the file defining the class
is imported. This will be fixed up for the first review UI.

Screenshot support has moved entirely over to this.

While not complete, this is a good base for the feature, and should get
us started so that students can start poking at code, and we can start
fine-tuning the support.

Verified that screenshot review still worked as it did before.

Tested with a branch for image review. The basics worked, except our
JavaScript really is not built to make image review outside of screenshot
classes work very well.