Use monospaced font for CodeMirror editor.

Cursor position gets off with non-monospaced font.
STUD-1456.

This bug does not exist for our CodeMirror usage in Problems and HTML because we already use a monospaced font there. I think the bug was introduced in our CodeMirror upgrade, and it may be particular to certain browsers (both Nimisha and I have seen it).

@marcotuts and @nasthagiri Please review.
@marcotuts is there a more attractive monospaced font?