user-import additional-fields

This allows to provide additional csv columns in user_import
and use them for replacing {0}, {1}, ... in the invitation mail
use overlay format with htmlfill (fixes #580)
Fix poll tiles if not in instance

This is a fixup of 3b97349dcefb652555aff99ee3cc7a3696af8680 and
0da4579e2e11c3d435d64c0e2473cb9298ebff68.
Fixup, in case there aren't additional fields
Merge pull request #582 from xi/feature-user-import-additional-fields

user-import additional-fields
Fix typo
fixup c83f765
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
Merge pull request #584 from xi/feature-htmlfill-overlay-format

use overlay format with htmlfill (fixes #580)
form overlays

Whan an overlays only contains a single form this allows us to

-   close the overlay on cancel
-   reload the whole page (without overlay) on submit
add style for loading submit button
replace overlay_target by overlay_type

needed by next commit
fix triggering form overlay from query params
use form overlay on sectionpage edit
cross browser fix