PLA-261 -- incorrect namespaces showing in file search

QA found a defect where non-file namespaces were showing in file search. This was because NS:6 was one of the query clauses included in our gradient "must match" clause. We address this problem in two ways:

1) Add the "required" operator to the first-order clauses (e.g. wiki ID and namespace), and move this logic up to the abstract class, since it's shared across multiple query services. 
2) Add namespaces to filter queries, mostly for safety.