explicitly set contentsCss in ckeditor config

ckeditor figures basePath from the location of ckeditor.js specified in
<script>'s src by default, but if we are in production mode ($DevelMode is
false), since ckeditor.js is also appended into /NoAuth/js/squished-...js,
ckeditor fails to find the correct basePath.

though we also set basePath before calling ckeditor, it's too late for
contentsCss url, which is a bug of ckeditor and fixed in 4.3.2

see also #29780 and http://dev.ckeditor.com/ticket/11350
Merge branch '4.2/ckeditor-contents-css' into 4.2-trunk