Before this change webpack started compiling before the cli version
if the app was compiled in doCliStuff.

Not sure how but this was causing webpack to send the cli version
of the app to the browser when main.js was requested. This caused
the app to not render anything (because it was a worker app) and
not show any errors.

The change just waits until the cli finished compiling to only
then move the .elm files to the /gen folder, triggering webpack
to recompile using the correct (ui version) of the app.