This patch switches out CodeMirror for a custom build based on version

5.5.0. A patch has been applied that allows Markdown and

GitHub-flavoured markdown editors to override the styles of any

generated elements. We use this to fix the previous workaround of code

blocks being rendered as comments (i.e., they were italicized) and to

fix the styling on lists (they are no longer styled).
NB: Once the patch is accepted upstream and a new release is done

(which should be 5.7.1) we will switch back to using the official

source tree.

Ran CodeMirror unit tests before building.
Manually verified that code blocks are not italicized.

Manually verified thats lists are not styled.