do_delete_user: Make the replacement dummy user inactive.

Otherwise, the dummy user will show up in the user list in the right
sidebar.

zerver/lib/actions.py

@@ -1228,6 +1228,7 @@ def do_delete_user(user_profile: UserProfile) -> None:
         password=None,
         realm=realm,
         full_name=f"Deleted User {user_id}",
+        active=False,
         is_mirror_dummy=True,
     )
     subs_to_recreate = [