Expand QA extended content to create sets driven from subgraph

[elrayle]

See QA Issue: samvera/questioning_authority#269

[elrayle]

@sfolsom @hudajkhan Can you review the proposed design?

[hudajkhan]

From the interface end, the proposed solution follows the same pattern as before (an array of properties) but now adds in the possibility of an element that is "sets" which includes multiple arrays where each array has properties that together define the same object/context node. I think we can work with that so am ok with the design at this point.

[zimeon]

@sfolsom -- would be good to understand which use cases / authorities we need this facility for