Modernize the display of the "Review request changed" boxes.

The "Review request changed" boxes haven't been touched since they were
first introduced back in 2006 or 2007. Since then, we've added file
attachments, improved the information available on diffs, introduced
Markdown, and added extension-provided fields. All of which the old
boxes did a terrible job at representing.

This introduces a complete overhaul of the design and capabilities of
these boxes. Every bit of information is now more detailed, making it
easier for people to see exactly what has changed at a glance.

Text area fields, like Description, are now rendered as Markdown, but
presented in a unified diff format. It's easy to see what changed in the
text, and to even see small changes like a corrected word within a line.
This works much like the diff viewer, except not side-by-side.

Simpler text fields, like Branch show the before and after, one on top
of the other, helping to see what changed. These are also presented in a
diff-like view, though much smaller. By placing one above the other,
instead of one to the side of the other, it's easier to see what has
changed.

Lists share the same design as the text fields, except there's one row
per removed entry and one per added.

Diffs are greatly improved. An uploaded diff still shows the revision, a
link to the diff, and the "Show changes" link. Along with that, it now
shows the raw insert/delete count, and a partial file listing, complete
with complexity icons. If the diff hasn't been viewed yet, the
complexity icons just show the proportion of inserts/deletes, but after
it's viewed and the lines are calculated, it'll show the same
information that the diff viewer shows.

File attachments now show the thumbnails for the files that were added
or removed. These are just like the ones on the review request file
listing, but without the editing capabilities. They do have reviewing
capabilities, though, so a user can immediately begin reviewing the
added files without going to the main file listing.

Fixes bug #2045