Previously, opening and closing a Markdown editor would mark the field

as dirty and, in the case of the review request editor, would create a

new review request draft. The reason this occurs is that CodeMirror's

isClean does not work the way we want (or it lies), so we check for

dirtiness ourselves by comparing the initial value with CodeMirror's

reported value.

Ran JS tests.
Verified that editing either the text or the "Enable Markdown" setting

creates a draft while just opening and closing the editor does not.