models: Add denormalized .realm column to Message.

This commit adds the OPTIONAL .realm attribute to Message
(and ArchivedMessage), with the server changes for making new Messages
have this set. Old Messages still have to be migrated to backfill this,
before it can be non-nullable.

Appropriate test changes to correctly set .realm for Messages the tests
manually create are included here as well.

zerver/tests/test_message_topics.py

@@ -40,6 +40,7 @@ class TopicHistoryTest(ZulipTestCase):
             message = Message(
                 sender=hamlet,
                 recipient=recipient,
+                realm=stream.realm,
                 content="whatever",
                 date_sent=timezone_now(),
                 sending_client=get_client("whatever"),