Add facet for subsets (slims)

[dosumis]

At the synapse annotation jamboree at the Broad. The biologists here really want to be able to get a view of the data based on the slim we plan to define for this work. Presumably it would be relatively easy to add a facet allowing users to choose terms with with a specified subset annotation.

[cmungall]

OK, so this would be an additional facet on the associations and bioentity views, like the existing 'indirect annotation' facet, but showing only classes in the subset? No map2slim would be performed.

This would potentially be confusing for most slims which span the width of the ontology, such as golsim_generic. Most terms will be in this slim, or a descendant of it. I assume the synapse slim will allow us to restrict purely to terms that are related to synapsey things, or descendants?

(note: I edited my original comment, in which I requested a subset filter on the ontology class view. This already exists!)

[kltm]

Depending on the exact use case, it may be technically covered by #69.