search: facets per collection

Allows to configure independently facets for every collection

Flask-Admin module available at /admin/facetcollection/

Matching fixtures for demosite: https://github.com/inveniosoftware/invenio-demosite/pull/21
Adaptation for inspire: https://github.com/inspirehep/inspire-next/pull/21

There is no need to recreate DB. The facets added to the default collection with id '1' will be displayed in other collections which doesn't have the facets set defined.

You need to run the updater to update your DB.

Alters: https://github.com/jirikuncar/invenio/pull/242
I've tried already and it doesn't work. At facets there is the relation defined, and it must be defined at every foreign key. At the other side, in this case at Collection, there must be backref. No changes here.