Mangling mail archive "flat" links

Hello,

It would be very nice if the archives didn't corrupt URLs like the one
at the bottom of this message:

https://www.postgresql.org/message-id/CA%2BhUKGJ8NRsqgkZEnsnRc2MFROBV-jCnacbYvtpptK2A9YYp9Q%40mail.gmail.com

I peeked in pgfilters.py and saw that there is a regular expression
designed to avoid mangling archives URLs, but it apparently doesn't
match the "flat" ones.

Thanks!