Issue due notification emails are threaded incorrectly

When receiving issue due email notifications and they are threaded by GMail, the email shows up in the wrong order.

It shows up like this:

Notice that the issue due email which is the latest one shows up first in the thread instead of last.

I think this is due to it having a Message-ID of issue_xxxx@gitlab.com and all the other notifications related to the issue have In-Reply-To or References to that message id.

I think this should be similar to milestone change notifications where we have:

Message-ID: <5b797ce61435882353b3bf2e9a72a82d@gitlab.com>
In-Reply-To: <issue_22652906@gitlab.com>