Render broken link in HtmlEditorField

Regression: 2.4 had this in the default cms/css/editor.css,
which got moved to the simple theme at some point, where this styling was removed.

See http://db.tt/bfEaeQby. The file was already searched for in this location through LeftAndMain, just never found. Its not ideal to have another HTTP request in the CMS, but I don't see a way around this, unless we either hack up TinyMCE's own CSS files, or rely on the CSS being set in the current theme (quite unlikely).