Feature request: allow relative URLs for photoAlbumURL

And for albumDirURL for that matter.

I have a webserver that sits on different networks (depending on whether it's at home or uni) so can't use an absolute URL to refer back to gallery. I can work around this by manually changing config.php on each upgrade so the URLs are relative, but it's a pain, so it would be nicer if the configuration system allowed relative URLs for these fields.