New global per-field value selections

[theq629]

This is due to #168 but I'd like it to have its own issue since it may be useful for later changes.

In the storylinesidekeyclean branch Facets.FieldSelections now provides a way to get a selections interface object to manage selections on any data field, synchronized to appropriate constraints. We instantiate Facets.FieldSelections at the top level (index.html) and pass it out as needed. The selections work basically like abstract facets, without the UI attached. We then attach the facet UI to some of them. Currently the other use is for the storyline view to select non-facet entities (from manual queries).

This change is fairly small since changes for #168 and #183 had already isolated the necessary code.

A minor oddity is that right now constraints for fields with facets will be shown in the UI list with the titles from the config files, while constraints for other fields will just use the raw field name.