use CKEDITOR_BASEPATH instead to specify ckeditor's basePath

ckeditor figures basePath from the location of ckeditor.js specified in
<script>'s src, 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.

we can get around it by using CKEDITOR_BASEPATH global variable instead.

see also #29780 and #29987
Revert "explicitly set contentsCss in ckeditor config"

This reverts commit 0a3badea2d49615f47d971e9d94ab9b11a87afe0.

since we explicitly set ckeditor's basePath by global var CKEDITOR_BASEPATH,
this commit is not necessary any more
Merge branch '4.2/ckeditor-base-path' into 4.2-trunk