Backend: Add ability to filter associations based on qualifiers

[kevinschaper]

I'm not sure how best to offer this.

The most literal approach would be simply to add each qualifier field to the list of association filtering params.

The actual use case was finding associations with any onset qualifier, which actually seems pretty reasonable, but we would need to have something specific like onset_qualifier=Any that we translate into onset_qualifier:[* TO *] in solr or onset_qualifier is not null (or maybe empty string in our sqlite db still?)

[kevinschaper]

The association table api endpoint also needs the ability to facet and filter on qualifiers like onset and frequency