Add solr schema version to buildout to force rebuild
Add field 'hidden' to solr schema
Add hidden instances to solr and use solr to filter them
Allow non-adhocracy classes as facet entity types

Previously, facet items always referred to adhocracy entities which can
be resolved through `model.refs`.

This commit allows to define facets with custom facet item classes. To
do this, such a class must subclass `SolrFacetItem`.
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
Rename InstanceStateFacetItem to TranslatedSolrFacetItem ...

... in order to make clear that this is a generic class which can be
used by multiple facets.
Merge pull request #587 from nidico/feature-show-hidden-instances

Show hidden instances to global admins