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

tests/regressiontests/admin_views/models.py

@@ -566,3 +566,9 @@ class AdminOrderedAdminMethod(models.Model):
 class AdminOrderedCallable(models.Model):
     order = models.IntegerField()
     stuff = models.CharField(max_length=200)
+
+class Report(models.Model):
+    title = models.CharField(max_length=100)
+
+    def __unicode__(self):
+        return self.title