Update from CodeMirror 5.8 to 5.26.
Review Request #698 — Created June 12, 2017 and submitted
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/