By default, Python's json module can't encode a set. This hasn't

historically been a problem for us, but with the introduction of code

safety checks in Review Board, we're now trying to encode sets in

responses.
This change adds formal support for a set in all of our JSON

serializers, including that for the API. The set is turned into a

sorted list of items.

Unit tests passed.
Verified that I could retrieve API results containing encoded sets.