migrations: Add noop reverse_code for migration 0419.
migrations: Add noop reverse_code for migration 0419.

This migration is naturally one that should be a noop when
downgrading, so declare it as such.

zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip