Use symlinks instead of copying site files

The copying from ```sites-templates``` to ```sites-enabled``` is duplication and a symlink can be used instead. This means the mounted data is used, rather than growing the container.

This will be useful in the case of sites with loads of files or very big files. My use case is self-hosting a vagrant box which is several hundred MB. I don't want to copy this from the mounted volume into the container so a symlink is better.