Instantiate CodeMirror widgets on demand only.
This lightens MarkdownEditor to only instantiate a CodeMirror wiget as

needed. MarkdownEditor will now create it when show() is called, or when

the inlineEditor begins editing. It will then go away when hidden or

when editing is finished. It should reduce the memory footprint a little

bit. Though it's a bit hard to tell what the savings is, it looks like

it may be as much as 8-10MB.

Tested opening, saving and canceling editors.


Checked the DOM inspector before, during and after, and saw that the

  CodeMirror widgets only existed while the editor was in edit mode.


Tested review requests, the review dialog, and comment dialogs.