www: do not double-clean Message-IDs from internal DBs
Ensure we usually strip one level of '<>' from Message-IDs,
since our internal SQLite, Xapian, and SHA-1 storage all
assume that.
Realistically, we screw up if somebody has '<<' or '>>',
but those are screwed up mail clients and we can deal with
it another time. Currently, this means some messages with
'>>' in References or Message-Id are not handled correctly,
yet, but we match the behavior of Mail::Thread in keeping
the extra '>'.