search: configuration of facets per collection

* Adds FacetCollection model with db upgrade.

Signed-off-by: Kamil Neczaj <kamil.neczaj@cern.ch>
search: flask-admin module to configure facets

* Adds Flask-Admin module available at /admin/facetcollection/.

Signed-off-by: Kamil Neczaj <kamil.neczaj@cern.ch>