Change http:// URLs to https:// wherever possible

Most debian.org URLs are nowadays reachable via https. As Chromium is
showing errors about unsecure connections for form data, all the URLs
that can be reached via https now, have been changed.

Signed-off-by: Martin Zobel-Helas <zobel@debian.org>
Signed-off-by: Raphaël Hertzog <hertzog@debian.org>