Fulltext search refinements

* enable empty query `q=&...` to return all visible objects (or counts
  therof)
* include `NULL` in list of visible contexts
* enable `counts_only=true` query parameter to return the count of
  matches per type
* deduplicate `results.entries`
Merge pull request #79 from reciprocity/feature/search-refinements

Fulltext search refinements