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.

zilencer/management/commands/populate_db.py

@@ -1008,6 +1008,7 @@ def get_recipient_by_id(rid: int) -> Recipient:
 def generate_and_send_messages(
     data: Tuple[int, Sequence[Sequence[int]], Mapping[str, Any], int]
 ) -> int:
+    realm = get_realm("zulip")
     (tot_messages, personals_pairs, options, random_seed) = data
     random.seed(random_seed)
 
@@ -1021,7 +1022,7 @@ def generate_and_send_messages(
     # We need to filter out streams from the analytics realm as we don't want to generate
     # messages to its streams - and they might also have no subscribers, which would break
     # our message generation mechanism below.
-    stream_ids = Stream.objects.filter(realm=get_realm("zulip")).values_list("id", flat=True)
+    stream_ids = Stream.objects.filter(realm=realm).values_list("id", flat=True)
     recipient_streams: List[int] = [
         recipient.id
         for recipient in Recipient.objects.filter(type=Recipient.STREAM, type_id__in=stream_ids)
@@ -1053,7 +1054,7 @@ def generate_and_send_messages(
     messages: List[Message] = []
     while num_messages < tot_messages:
         saved_data: Dict[str, Any] = {}
-        message = Message()
+        message = Message(realm=realm)
         message.sending_client = get_client("populate_db")
 
         message.content = next(texts)