Markdown list fixes

I kept the commits (outlined below) related to different problems separated in case you didn't want to pull all of them in.

### [markdown] Improve highlighting of lists.

This fixes #1276. It also improves highlighting of nested list items. It uses multiple classes to differentiate between nesting levels. It uses 3 classes instead of 2 in order to avoid confusion between 3rd and 1st level (which is quite possible, due to Markdown's silly handling of indentation). Using 3 classes also brought a couple bugs from the original implementation to my attention (which are fixed as well).

### [markdown] Fix test that was changed in a2ad87f.

When the tests were changed to the new format (which I love, btw), an extra new line was lost in translation. This just adds it back in.

### [markdown] Improve highlighting of blockquotes.

Blockquotes suffered from many of the issues lists had. For example, code highlighting was hidden, due to CSS specificity, and nested comments had the same highlighting. This commit fixes the issues.