Fixed #6218 -- Made MEDIA_URL and STATIC_URL require a trailing slash to ensure there is a consistent way to combine paths in templates. Thanks to Michael Toomim, Chris Heisel and Chris Beaven.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15130 bcc190cf-cafb-0310-a4f2-bffc1f526a37