Finish the haskell-magic-wormhole "app versions" branch somehow

Version negotiation is supported on a branch of haskell-magic-wormhole. Merging it might be slightly tricky because having it work with reflex-platform and modern library versions may require some gymnastics.

The branch is small right now but we don't want to end up stuck on an ever-growing long-lived branch that becomes a de-facto fork.

So, figure it out.

See https://github.com/LeastAuthority/haskell-magic-wormhole/pull/68