markdown: don't double-escape ">" and "<" in code blocks

See #416.

I removed the `sanitize_markdown` method call, which will gsub all "<" and  ">" with their HTML entities.

When Markdown parsed the file, it kept those entities intact, _unless_ they were in a &lt;pre&gt; or &lt;code&gt; block. Then it escaped them again, so they were doubly-escaped.

I also changed a test which incorrectly expected the doubly-escaped output.

The other tests pass, but does this introduce a XSS vulnerability I'm not seeing?