Lock Mustache helper contexts to response sets

See https://code.bioinformatics.northwestern.edu/issues/issues/show/2744.

The scenario that demonstrates this is pretty janky and awkward, but:

1. fails as expected (i.e. with incorrect survey rendering) without the necessary changes in place
2. passes with the necessary changes in place

Part of the awkwardness stems from crazy ways to invoke webmocked PSC connections.  All that said, this is a review, so changes to clarify the test process are welcome.

The huge change count in `instrument_context_spec.rb` is due to baa48c289e404052105576b2d64e14e5d3eb15fc.