new optional https_url config value

Hey,
I am suggesting a simple config addition.

When I started using https on my nginx, I decided to let Monitorix bind on 127.0.0.1 and use nginx as reverse-proxy. That worked fine, but Monitorix cgi kept using http:// scheme for its urls ($config{url} variable all over the Monitorix) which makes sense as it don't know I am proxying http requests which are https when looking at it from the outside world.

To illustrate - here is my setup:
BROWSER <-https-> NGINX (0.0.0.0) <-http-> MONITORIX (127.0.0.1)

This way I can protect monitorix and other similar locally-listening servers via nginx auth and https transfer.

The proposed change adds a new optional variable https_url which can be added to Monitorix confix file. When it's https_url = y, Monitorix will use https:// scheme for all links even when the internal monitorix server is listening for HTTP requests. All existing functionality for HTTPS CGI detection etc remains. I have tested all three cases (https_url se to y, to n and not set). Worked fine.

Alternatively it could be solved by using relative URLs everywhere (respecting base_url and url from config)....

Cheers,
Mario