Fixes these `npm install` warnings:
> deprecated babel-eslint@10.1.0: babel-eslint is now @babel/eslint-parser. This package will no longer receive updates.
> deprecated eslint-loader@4.0.2: This loader has been deprecated. Please use eslint-webpack-plugin
Cherry-picked from starter-kit commit 0e608d562a
Taken rules from Cockpit, which have been proven to be effective for
finding bugs and keeping clean code, and sensible (not taking
unreasonable effort to satisfy).
Fix code layout in app.jsx accordingly.
Closes#39
