Lighten the deleted lines color in the diff viewer.

The deleted lines color in the diff viewer was still a lot darker than
the insert and replace lines, and the contrast with the move flags
weren't great. This lightens it a bit more, making it appear a lot
closer to the other colors, and making it easier to read.
Improve the style of the move indicator flags.

The move indicator flags were very noisy things. There was a flag for
every moved line, which made it harder to see that a range was moved as
a whole, if two ranges were right up against each other.

This changes the design to only have a flag for the first line in a
range, and not subsequent lines. Those are indicated by a tail on the
flag, or a flag pole perhaps.

This reduces the visual noise considerably, at the expense of only
allowing the first row to be clickable to jump lines. That's a necessity
due to the limitations in how we can represent this style in CSS (as we
cannot properly represent each row of that flag pole as an element with
the proper height in a TD without javascript).
Document Markdown support.

This change adds some relatively thorough documentation for using markdown.
The bulk of this is a page called "Using Markdown" which gives a quick overview
of the basics of markdown syntax for the things most people will care about. I
haven't made this an exhaustive document, since the pages on daring fireball
and github serve that purpose. Instead, this should hopefully be a nice quick
reference for the basics of markdown syntax as interpreted by Review Board.

This is linked to from all the various places where markdown can be used.

While I was in here, I updated everything to capitalize "Markdown", including
in the MD review UI docs, and capitalized the terms in the glossary (which
looks much better to me).

Testing done:
* Generated HTML docs and read through everything.
* Checked cross-linking.

Reviewed at https://reviews.reviewboard.org/r/4834/