Integrate CodeMirror with linting

[helen]

WordPress core file editors now use CodeMirror with real-time linting and a pre-save "are you sure" checkbox, which allows users to save things with suspected errors if they really really want or need to. Ads.txt would be a great place to also use this functionality as well as UI.

• Implement CodeMirror for the editor (Add codemirror, first pass #10)
• Linting - parser and errors (if this can be easily kept in sync with PHP that would be best)
• Pre-save AYS
• Use more of core's built-in code editor wrappers as sensible (JS and PHP, e.g. wp_enqueue_code_editor()).

Does syntax highlighting make any sense? Seems like overkill, probably.