improve dbux-projects webpack'ing

# Webpack Config

* [ ] #621
* [x] add support for `babel-loader` -> `cacheDirectory`
* [ ] consider enabling it by default (e.g., we enabled it explicitely in `react-tic-tac-toe`)
* [x] add `pw` and `pb` via [rule `test`, `include` and `exclude` options](https://webpack.js.org/configuration/module/#rule-conditions)
* [x] test all of the above with `react-tic-tac-toe`
* -> encoding of click event data takes `21s`
* [ ] monitor for Webpack build errors
* add error hook to `webpack.config.dbux.base.js` - [explanation](https://medium.com/finnovate-io/make-webpack-exit-on-compilation-errors-16d2eec03391#b197) (NOTE: V4 only)
* use [webpack's Node API: compiler.watch(options, callback)](https://webpack.js.org/api/node/#watching)?
* [ ] while executing `startWatchModeIfNotRunning`, show a webpack-specific message in progress bar
* (so as to let users know what is taking so long)
* [x] Q: when switching from one exercise to another, and they are of the same project: can we **not** stop and then re-start webpack?
* → Nah, too risky.

# UI
* [ ] add a button to reflect webpack status for webpack-enabled projects
* button can also be used to stop (or otherwise control) webpack