https for website, bug tracker and mailing list server

[Olly Betts]

The Survex website, bug tracker and mailing list server are now
available via https:

https://survex.com/
https://trac.survex.com/
https://lists.survex.com/

Thanks to James Aylett for doing the necessary setup on the server, and
to https://letsencrypt.org/ for the certificates.

Currently https is forced for trac.survex.com (because the traffic to
that includes passwords), but not for the other two sites.  At some
point we'll likely force https for all of them, but meanwhile I'd
recommend using https for downloads, especially if you don't check the
signatures or checksums.

Please let me know if you encounter any problems.

Cheers,
    Olly