Use assets bundler and proper dependency manager

[blazeu]

See https://medium.freecodecamp.org/javascript-modules-part-2-module-bundling-5020383cf306

TL;DR

• Having many <script> and <style> tags sucks and makes loading slow
• Combine all assets into a single .js & .css file (Webpack preferred)
• Since we're probably gonna use ES2015 modules to import modules/scripts, we can write all the codes in ES2015+ and have Babel + Webpack integrations to transpile the code into current version of JavaScript that is widely supported by current browsers
• Yarn/NPM(v5) is currently the recommended dependency manager for front-end (previously only for backend Node.js)