Fix EZP-20305: Be able to link from one SiteAccess to another

https://jira.ez.no/browse/EZP-20305

**DO NOT MERGE**

This PR makes it possible to generate links from one SiteAccess to another.

Example:
```jinja
<h1>
    <a href="{{ url( location, {"siteaccess": "foo"} ) }}">Some Title</a>
</h1>
```