Allow 3+ way queries in CAM-KP Frontend

[gaurav]

At the moment, we can ask arbitrary queries in the form (if subject_or_object=A AND if subject_or_object=B ...). But an important use-case is being able to ask "how are A, B and C related?" -- we want to be able to say "we don't have anything on all three, but here are three pathways that talk about A & B and here are two that talk about A & C". Not sure how to implement that, but I wanted to track that in its own issue.