Fixed #12481: Updated admin validation code to not reject non-editable fields...

Fixed #12481: Updated admin validation code to not reject non-editable fields in readonly_fields, since these are handled fine by the display code itself. Thanks lashni and Alex.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@12730 bcc190cf-cafb-0310-a4f2-bffc1f526a37