Rewrite monads from Polysemy to RWST

[ajkavanagh]

Although it was a nice experiment, I think it would be better to re-write the app to not use Polysemy and instead a conventional (set of) monad transformer stack(s). Although it is easier to test, there aren't that many tests (which is not necessarily a good thing!) and Polysemy seems to kind of just get in the way. I'm creating this issue to see how I think about this over time.

Certainly, threading Polysemy through Ginger was a right pain, and I don't particularly want to maintain that code anyway.