we need shell=True as on Windows, path resolution to find the yarn
"exectuable" (a shell script) is only performed by cmd.exe, not by
Python itself. as we are calling yarn with fixed arguments, using
shell=True is fine.
add pre-script to check if webapp was compiled or sources changed and auto compile.
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Co-authored-by: Bernhard Kirchen <schlimmchen@posteo.net>