Include server hostname in TreeView cache-key (fixes virtual hosting)

With the current caching of the TreeView portlet, the absolute URLs get cached with the first server name that a particular user visits the site with.

This can in some cases be `localhost`, for example if we connect through an SSH tunnel, and will therefore break virtual hosting.

This PR includes the server's hostname in the cache-key in order to address this problem.

The `get_hostname()` function is from https://github.com/plone/documentation/blob/master/develop/plone/serving/http_request_and_response.rst#hostname

/cc @phgross