Modern versions of CodeMirror bring a lot of bug fixes, API
improvements, compatibility fixes, and more. Along with upgrading, this
build bundles new syntax highlighting modes for JSX and Swift, and
includes a new plugin for optionally showing placeholder text for empty
fields (which may prove useful for guidance later in new review requests
or other fields).

Testing Done:
Tested all major editing and Markdown features in review request fields,
comments, and replies.

Reviewed at https://reviews.reviewboard.org/r/8990/