FIX: auto response email replies should not be accepted

[Fixes bug reported here](https://meta.discourse.org/t/auto-response-email-replies-to-digests-are-processed-as-new-topic/22564?u=techapj).

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[more info regarding `Auto-Submitted Header` here](http://www.iana.org/assignments/auto-submitted-keywords/auto-submitted-keywords.xhtml).