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/tests/test_auth_backends.py

@@ -892,8 +892,7 @@ class GoogleSubdomainLoginTest(GoogleOAuthTest):
         # Now confirm an invitation link works
         referrer = self.example_user("hamlet")
         multiuse_obj = MultiuseInvite.objects.create(realm=realm, referred_by=referrer)
-        multiuse_obj.streams = streams
-        multiuse_obj.save()
+        multiuse_obj.streams.set(streams)
         invite_link = create_confirmation_link(multiuse_obj, realm.host,
                                                Confirmation.MULTIUSE_INVITE)
 

zerver/tests/test_signup.py

@@ -1024,8 +1024,7 @@ class MultiuseInviteTest(ZulipTestCase):
         invite.save()
 
         if streams is not None:
-            invite.streams = streams
-            invite.save()
+            invite.streams.set(streams)
 
         if date_sent is None:
             date_sent = timezone_now()