populate_db: Limit user_profiles for private messages to zulip realm.

These are used for creating huddles and private messages (and some
UserPresence objects). It'd be really weird, and potentially create some
Messages that break our assumptions, for this to end up involving users
in multiple realms.
I believe currently this hasn't been happening, because when
this line runs, there are only users in "zulip" realm and system bots in
"zulipinternal" - but the query has been excluding bots already.

Still, this query should be explicit about grabbing users from a single
realm. This will also be helpful for the work adding the denormalized
Message.realm field - so that the realm of Message objects that get
manually created in generate_and_send_messages can be simply set to
"zulip" with confidence that it's correct.

zilencer/management/commands/populate_db.py

@@ -764,7 +764,9 @@ class Command(BaseCommand):
             ]
 
         # Extract a list of all users
-        user_profiles: List[UserProfile] = list(UserProfile.objects.filter(is_bot=False))
+        user_profiles: List[UserProfile] = list(
+            UserProfile.objects.filter(is_bot=False, realm=zulip_realm)
+        )
 
         # Create a test realm emoji.
         IMAGE_FILE_PATH = static_path("images/test-images/checkbox.png")