Overriding detected server port

When hosting Shaarli on a webserver behind a [sslh](http://www.rutschle.net/tech/sslh) instance, generated urls contains the actual webserver port (based on $_SERVER['PORT']) .

In such a config, the webserver may not be directly reachable from outside and using the detected port isn't relevent, as it will be the one on which sslh run which will be serving pages.

As a workaround I added a global ACTUAL_SERVER_PORT, as you can see [here](https://github.com/jcsaaddupuy/Shaarli/blob/master/index.php#L32) and [here](https://github.com/jcsaaddupuy/Shaarli/blob/master/index.php#L439) which is will be used in the serverUrl function.

In order to be coherent, I also made the RSS/Atom URL relatives (like the other ones) by removing the {$feedurl} variable in tpl/includes.html and tpl/page.header.html.