Correctly configure the codehilite Markdown extension.
Review Request #10586 — Created June 11, 2019 and submitted
We had a configuration option set for the Markdown
extension that referenced it by the old name rather than the module
path. This only worked on older versions of Python Markdown, and failed
on anything modern, resulting in differences in behavior depending on
the version being used. The module-based name is always safe, so it's a
simple fix to just use that one.
Unit tests pass against new versions of Python Markdown.