Updates Webpack to v2 and updates corresponding dependencies

[frometor]

incremental migration of Zappr from Webpack version 1 to version 2. Updates dependencies and reduces vulnerabilities. part of #543

[fokusferit]

@frometor do you need some help?
Ci says:

BREAKING CHANGE: It's no longer allowed to omit the '-loader' suffix when using loaders.
                 You need to specify 'null-loader' instead of 'null'. 

so I guess some integration work is necessary? I hope that it is not related to some webpack conf inside a dependency ^^

[frometor]

@fokusferit I know. I am pretty sure it is part of a webpack conf inside an outdated dependency.
webpack.server.js:

/**
 * Externalize node_modules.
 */
const nodeModules = () => (
  fs.readdirSync('node_modules').
  filter(x => ['.bin'].indexOf(x) === -1).
  reduce((modules, m) => {
    modules[m] = 'commonjs2 ' + m;
    return modules
  }, {
    'react-dom/server': 'commonjs2 react-dom/server'
  })
)