django-2.0: Don't assign directly to Many-to-Many field.

The old pattern of setting the value and then using .save() here has
been deprecated.  set() also saves the record.

zerver/views/registration.py

@@ -304,8 +304,7 @@ def prepare_activation_url(email: str, request: HttpRequest,
     prereg_user = create_preregistration_user(email, request, realm_creation)
 
     if streams is not None:
-        prereg_user.streams = streams
-        prereg_user.save()
+        prereg_user.streams.set(streams)
 
     confirmation_type = Confirmation.USER_REGISTRATION
     if realm_creation: