.CodeMirror is supposed to magically replace it. Some gadgets'
interface displays incorrectly without it.
Change-Id: I27e2b5b462aa445a13f50dc99af57b4bb0ad3067
Long-term todo:
* Performance will be poor on large pages due
to using a auto-height textarea which CodeMirror
doesn't optimise.
Change-Id: I16598fcdbeee51e6fae88376ec81f1c8552b383d
* Move CSS out of mediawiki.css which is for wikitext
highlighting rules.
* Account for wikieditor-ui adding wrappers even when
disabled.
Change-Id: I0fca693a6771ee1d790800c9afd5c7091fda20c1