Martin Brennan f26b8b448d
FIX: References header leading to broken email threading (#15206)
Since 3b13f1146b2a406238c50d6b45bc9aa721094f46 the email threading
in mail clients has been broken, because the random suffix meant
that the References header would always be different for non-group
SMTP email notifications sent out.

This commit fixes the issue by always using the "canonical" topic
reference ID inside the References header in the format:

topic/TOPIC_ID@HOST

Which was the old format. We also add the References header to
notifications sent for the first post arriving, so the threading
works for subsequent emails. The Message-ID header is still random
as per the previous change.
2021-12-08 08:14:48 +10:00
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