Webpack 5 support

https://webpack.js.org/migrate/5/

• update webpack 4 to latest version
• update plugins to latest version supporting webpack 4
  • css-loader
  • file-loader
  • html-webpack-plugin
  • http-proxy-middleware
  • postcss-loader
  • inject-loader (doesn't support webpack 5)
  • mini-css-extract-plugin
  • karma-webpack
  • raw-loader (doesn't support webpack 5)
  • sass-loader
  • url-loader
  • vue-loader
• make sure no warnings
• update webpack to 5