confirmation: Add an index on content_type, object_id.

This makes lookups of `.confirmation`, or joins through
`confirmation__` as we do in the `/json/invites`, actually fast.  See
https://code.djangoproject.com/ticket/23435

(cherry picked from commit fcb04598f0)

confirmation/models.py

@@ -231,6 +231,9 @@ class Confirmation(models.Model):
 
     class Meta:
         unique_together = ("type", "confirmation_key")
+        indexes = [
+            models.Index(fields=["content_type", "object_id"]),
+        ]
 
     @override
     def __str__(self) -> str: