use both Date: and Received: times

We want to rely on Date: to sort messages within individual
threads since it keeps messages from git-send-email(1) sorted.
However, since developers occasionally have the clock set
wrong on their machines, sort overall messages by the newest
date in a Received: header so the landing page isn't forever
polluted by messages from the future.

This also gives us determinism for commit times in most cases,
as we'll used the Received: timestamp there, as well.

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