Use CodeMirror as the new text field for editing Markdown.
The big problem with Markdown input was that it was too easy to

unintentionally add characters that would render text differently, such

as _, or `. While it wasn't a big deal for things like the review

request description fields, it was a problem for comments, as you

wouldn't know until you published how they look.
To fix this, I've added CodeMirror as a dependency, as integrated it

with all our edit fields. Now, instead of a textarea with just plain

text, there's a syntax-highlighted editor that otherwise behaves just

like a textarea.
CodeMirror itself is hidden behind a new view, MarkdownEditorView. This

is the view used for all text fields now. It provides a convenient way

to integrate in with inlineEditor as well.
All the benefits we had before are still there. CodeMirror does a good

job of feeling native on each platform, and it can autogrow the text

fields to fit the content, just like textarea with autoSizeTextArea.
What it doesn't do is syntax-highlight any code added in the Markdown

text (like Python). It could, but it means adding a bunch of

dependencies for each highlighter, which are bulkier than those in

google-code-prettify (due to also being tokenizers). Even with those

added, it requires an explicit ```language block in order to highlight

them.
It will, however, highlight that they are code blocks, so we have that

at least.

Tested this on Chrome and Firefox.
I tested the Description and Testing done fields on review requests,

change descriptions, the body top/bottom and comment fields on the

Review dialog, the comment dialog, and review replies.
I also tested interaction in the comment dialog, such as resizing the

dialog and checking that the dirty status was correct.
Tested that everything loaded and saved correctly.