Avoid mixed content when cgiurl is https but url is not
Add reverse_proxy option which hard-codes cgiurl in CGI output

This solves several people's issues with the CGI trying to be
too clever when IkiWiki is placed behind a reverse-proxy.
In html5 mode, generate a host- or protocol-relative <base> for the CGI

This increases the number of situations in which we do the right thing.
Document another fix
more fixes