For redundant messages sharing Message-IDs, the link to solver
(/$INBOX/$OID/s/) was going up too many levels for /$INBOX/$MSGID/
when there were multiple messages sharing the same $MSGID.
Unfortunately, redundant messages are common with /all/
due to signature trailers. So dynamically assigning {-spfx}
is tricky and error prone from counting `/'.
So simplify the code a bit by setting {-spfx} once per HTTP
request, instead of every single message.