Fixed #13038 -- Ensured that readonly fields in the admin have their name...

Fixed #13038 -- Ensured that readonly fields in the admin have their name added as a CSS class. Thanks to andybak for the report, and javimansilla, fisadev and fgallina for their work on the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12922 bcc190cf-cafb-0310-a4f2-bffc1f526a37