JS: can't build programs with newer Cabal

Since https://github.com/haskell/cabal/commit/8d402c7b2900ebeb2c320e40cf4c16338232eb43 Cabal passes -this-unit-id to GHC when it builds a program. This broke the JS linker which still assumes that mainUnitId is used.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information