[4.x] Add ability to set site on the mount_url tag

[aerni]

This PR adds the ability to specify the site to use on the mount_url tag. This is helpful in a multi-site environment, where the collection isn't available in the site the tag is used on.

Example
I've got two sites, gounity and gomagazin. The events collection only exists in the gounity site. I want to link to the events collection mount from the gomagazin site. You can now do this by setting the site on the tag.

{{ mount_url:events site="gounity" }}

Without this PR, the tag would return null as the events collection isn't available in the gomagazin site.