Ability to display facets and facet counts on the table view.

Facets can be specified in the URL with ?_facet=column&_facet=othercolumn or the default facets for a table can be set using a new "facets": [...] property in metadata.json

• Implement ?_facet=
• Implement metadata.json facets key
• Design for how facets should be presented
• Facets should be able to toggle off as well as on
• Expand labels for facets that are foreign keys
• Suggest potential facets (if we can do so within a tight time limit)
• Documentation