Instead of overriding rules we don't want by matching the test exactly,
we now explicitly delete rules we don't want before adding any of our
custom rules.
Match the versions of Webpack, Babel, and related plugins as used in
scratch-gui. That plus a few other tweaks means that this repository
builds the same with or without `npm ln scratch-gui`, and also means
that all syntax used in scratch-gui should build the same here.
