models: Remove type prefixes from __str__ values.

The Django convention is for __repr__ to include the type and __str__
to omit it.  In fact its default __repr__ implementation for models
automatically adds a type prefix to __str__, which has resulted in the
type being duplicated:

    >>> UserProfile.objects.first()
    <UserProfile: <UserProfile: emailgateway@zulip.com <Realm: zulipinternal 1>>>

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/tests/test_message_send.py

@@ -1645,9 +1645,9 @@ class StreamMessagesTest(ZulipTestCase):
         self.send_stream_message(sender, "Denmark", content="whatever", topic_name="my topic")
         message = most_recent_message(receiving_user_profile)
         self.assertEqual(
-            str(message),
+            repr(message),
             "<Message: Denmark / my topic / "
-            "<UserProfile: {} {}>>".format(sender.email, sender.realm),
+            "<UserProfile: {} {!r}>>".format(sender.email, sender.realm),
         )
 
     def test_message_mentions(self) -> None: