Extend FacetMatrix to combine with facet drill down

[cmungall]

See also monarch-initiative/monarch-app#697

Currently we choose two facets and see a generated grid with static values for intersections for each axis.

Can we also drill down dynamically using another facet?

[kltm]

I don't think anything like this would be particularly usable until the Solr 5.x transition (berkeleybop/bbop-js#16). It's already a bit marginal, which is (one of the reasons) why it has not yet escaped labs.

[kltm]

Hrm, looking at it again, it probably wouldn't be that hard (I had forgotten that we spun it out into it's own "application" page), just slow in some cases. Assuming that you pop up a shield on every filter change, you might be able to squeeze something useful out of that without too much trouble.