Add instance hidden state facet for global admins

NOTE: This requires to reindex all instances in solr by doing:

    bin/paster index INDEX instance -c etc/adhocracy.ini
Add instance state (open vs frozen) facet

As a bonus of the last commit (instance hidden state), this adds the
possibility to filter instances by state (open vs frozen) to all users.

This is done right now, as users need to reindex their instances in solr
anyway by doing:

    bin/paster index INDEX instance -c etc/adhocracy.ini