Add a config option to FORCE_HTTPS_LINKS

Add a config option to FORCE_HTTPS_LINKS

In situations where SSL is terminated at the load-balancer, we cannot
rely on guessing the scheme based on whether patchwork itself was
accessed via http or https, since the last-leg is always going to be
done over http.

Unfortunately, wrongly using http:// URLs results in unusable
.pwclientrc files, since xmlrpc does not handle http->https redirects
and instead displays a traceback.

This change introduces a FORCE_HTTPS_LINKS option, which forces
pwclientrc links to always return "https" regardless of how the project
itself is accessed.

It appears that the http/https check is currently only used for
generating pwclientrc -- a lot of other places seem to hardcode
"http://" and rely on the server to transparently upgrade the
connection. This is not a secure approach (it allows for MITM and
SSL-Strip attacks) and therefore all places currently hardcoding
http://{{site.domain}} and similar should be switched to using the
"sheme" variable, the same as done for generating pwclientrc files.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>