change_realm_subdomain: Revoke user sessions.

If we don't revoke sessions, this can lead to issues when we import a
realm (e.g. from Slack) into the original subdomain for the same
organization. Because then, users who already had sessions before the
move end up stuck:
- when they try to open Zulip on the original subdomain, which now has
the imported instance, they get redirected to the moved subdomain due to
their pre-existing session cookie.
- they can't even log out to get rid of the bugged cookie, because the
account they're logged into in that session is now on the moved
subdomain.

zerver/actions/create_realm.py

@@ -18,6 +18,7 @@ from zerver.lib.bulk_create import create_users
 from zerver.lib.push_notifications import sends_notifications_directly
 from zerver.lib.remote_server import maybe_enqueue_audit_log_upload
 from zerver.lib.server_initialization import create_internal_realm, server_initialized
+from zerver.lib.sessions import delete_realm_user_sessions
 from zerver.lib.streams import ensure_stream
 from zerver.lib.user_groups import (
     create_system_user_groups_for_realm,
@@ -94,6 +95,9 @@ def do_change_realm_subdomain(
         )
         do_add_deactivated_redirect(placeholder_realm, realm.url)
 
+    # Sessions can't be deleted inside a transaction.
+    delete_realm_user_sessions(realm)
+
 
 def set_realm_permissions_based_on_org_type(realm: Realm) -> None:
     """This function implements overrides for the default configuration