Show hidden instances to global admins

Until now, it is possible to hide instances through the UI, however to find them again, the instance key needs to be known.

This PR does:

* All instances are added to Solr (not only non-hidden instances).
* The `hidden` attribute is also added to Solr.
* Global admins see all instances, while normal users only see non-hidden instances in the list.
* A facet *instance state* is added for global admins.

This also adds some helper code

* to make sure the solr schema is updated
* to make it easy to use non-adhocracy-entities as facet items.