Changes for new webinterface

[anfelor]

Hi @makarius, hi @adelon,

I have recently adopted the current Naproche master branch for the new webinterface at https://naproche.github.io/
For this I have made the following changes:

1. GHCJS does not support the network package. Unfortunately, this means that it is necessary to split Naproche into three different packages: app, Isabelle and src. The parts of the Isabelle sources that do not use network are now found under src/Isabelle, while those that use network are under Isabelle. Last year, I was able to keep the Isabelle sources together in my dev branch by creating an Isabelle package that shared its sources with the naproche package. But this is not possible here, as the sources now describe common types like Bytes and GHC would get confused if Bytes from the Isabelle package is the same thing as Bytes from the Naproche package.
2. I defined common APIs like MonadFile, MessageExchangeContext and RunProverContext that abstract from these system calls. There is a Console context with a basic implementation in src/ itself and a PIDE context in app/ (which depends on Isabelle/).
3. I made some small changes so that the project would work for GHC, like removing the use of the new ImportQualifiedPost extension and adding a MonadFail instance in the parser.