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/auth.py

@@ -92,8 +92,7 @@ def maybe_send_to_registration(request: HttpRequest, email: Text, full_name: Tex
             del request.session["multiuse_object_key"]
             request.session.modified = True
             if streams_to_subscribe is not None:
-                prereg_user.streams = streams_to_subscribe
-            prereg_user.save()
+                prereg_user.streams.set(streams_to_subscribe)
 
         return redirect("".join((
             create_confirmation_link(prereg_user, request.get_host(), Confirmation.USER_REGISTRATION),