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>

tools/coveragerc

@@ -10,7 +10,7 @@ exclude_lines =
     # Don't require coverage for test suite AssertionError -- they're usually for clarity
     raise AssertionError
     # Don't require coverage for __str__ statements just used for printing
-    def __str__[(]self[)] -> .*:
+    def __(repr|str)__[(]self[)] -> .*:
     # Don't require coverage for errors about unsupported webhook event types
     raise UnsupportedWebhookEventTypeError
     # Don't require coverage for blocks only run when type-checking