Use Rule.oneOf to resolve correct loader

Created by: Furizaa

This PR uses the oneOf rule to resolve loaders in webpack.config.prod.js. This will use the first loader that matches in the list of loaders provided by oneOf and enables us to remove the big exclusion list of the file-loader. The file loader will only be used if the module falls through all other loaders.

https://webpack.js.org/configuration/module/#rule-oneof

For some reason though I had to still preserve the js(x) exclusion in the file loader. Otherwise the build would break. This happens with or without the eslint preloader. Will have to investigate further why exactly this happens and why these unknown js(x) modules fall through to the file-loader.

The resolve now looks something like this (pseudocode):

[
  { 'eslint-loader', enforce: 'pre', test: 'javascript' },
  {
    oneOf: [
      { 'url-loader': test: 'images' },
      { 'babel-loader', test: 'javascript' },
      { 'style-loader', test: 'style' },
      { 'file-loader' },
    ]
  }
]