Python-Markdown 2.6 is the latest stable release, and features bug fixes
and extension capabilities we'll want to take advantage of. Djblets
features some new compatibility fixes for newer versions of
Python-Markdown. This change implements the Review Board side of this
upgrade.

All the Markdown renderers now make use of the new extension for
escaping HTML text, removing usage of the deprecated safe_mode argument.
It also updates all the extension listings to use the full module paths,
which will be required in Python-Markdown 2.7.

Unit tests pass.
Tested with some review requests making heavy use of Markdown. Compared

the results before and after the Python-Markdown upgrade.