Use Heist for the results page

This is a straightforward translation of the previous code. Translating pattern matches in Haskell to Heist templates was rather awkward and I'd appreciate any suggestions for improvement. (One thing that came into my mind was interpreting undefined variables in if tags as False by default in order to get rid of the defaultFlags in ResultsSplices, but would also make debugging templates including if tags more confusing.)

I've also changed the MonadHeistState class so that it only depends on MonadIO in order to be able to use it to broadcast results without depending on MonadSnap when a computation is complete.