Don't set width: 100% for all images in the review request.

A recent fix for thumbnail overflows resulted in every image on a review
request box being set to a 100% width. This affected the mimetype icons
for file attachments and any images in the text areas.

The other fix that was part of the commit introducing this appears to be
sufficient for the original overflow issue.
Improve the output for the Markdown UI.

The Markdown UI's line generator for rendered source didn't handle lists
very well, and had some other less-than-ideal rendering issues. We now
have a more specialized line emitter for rendered Markdown, which has
been moved to markdown_utils.py for use by other code.

On top of that, source was rendered using some non-Markdown lexer.
Pygments, it turns out, doesn't have a Markdown lexer, oddly.
ReStructured Text is really close, but not a complete match, and could
lead to some confusion. So, we're using the plain-text renderer for now.
Split up the review_detail.html template.

review_detail.html was a bit large and hard to follow. It's now been
split up a bit. The change and review boxes now live in included
templates in reviews/boxes/.

There's also some indentation cleanup in the templates.
Collapse the {% load %} tags in templates.

We had a lot of {% load %} tags in old templates that we could easily
collapse down. This improves template rendering speed (fewer nodes to
process) and saves memory for caching.

I was also able to get rid of some loaded modules we weren't even using
in some templates.
Don't show the expand icon if a header isn't expandable.

If multiple sections of collapsed context exist within a single function, the
headers listed in the collapsed sections will be the same function name, but
only the first one is expandable. Unfortunately, the "+" icon was still shown,
despite it not having an <a> tag around it.

Testing done:
Loaded a diff where I noticed this issue. Saw that the function name was still
shown, but the expand icon was gone.

Reviewed at https://reviews.reviewboard.org/r/5476/