Stopped unconditionally reversing admin model add/change URLs.

Starting with [16857] this could cause HTTP 500 errors when
`ModelAdmin.get_urls()` has been customized to the point it doesn't
provide these standard URLs.

Fixes #17333. Refs #15294.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17237 bcc190cf-cafb-0310-a4f2-bffc1f526a37