index concept and scheme uris; add to fts table

these changes restrict indexing to SKOS concepts; they will be indexed
if they have one of the following predicates:

- http://purl.org/dc/terms/identifier
- http://www.w3.org/2004/02/skos/core#prefLabel
- http://purl.org/dc/terms/title
- http://www.w3.org/2004/02/skos/core#altLabel'

this was done to restrict values to SKOS URIs so that we can ensure that
each entry has a URI and belongs to a specific concept scheme (e.g.
category)