ESLint, Jest and template strings

• Switched from EJS for templating to basic HTML using template string interpolation.
  This change allows for much more flexibility, i.e. with react-helmet
  and dynamically requiring content, such as styles.
• Introduces PostCSS to enable autoprefixer, since support of older browser (looking at you IE) is still important.
• Implement eslint and basic configuration.
• Implements basic test engine using jest.
• Improves production build by executing steps in parallel.