@anoe This MR revert the Datafield instance to not use the Generic-based derived FromJSON and ToJSON, in favour of some manual instance which won't change based on the changes to the data structure.

I have also written roundtrip tests for WithQuery and Datafield, and in the process I have discovered a bug with the JSON instances of FlowSocialListWith, which were not roundtripping (I have now fixed it).

I have also added a regression test to make sure that we can decode the JSON the frontend will send us into a valid Haskell type, failing if that's not the case. Hopefully this should catch future regressions.