Make Standards Mode compliant

[freethlua]

Previously the app was rendered in Quirks mode and switching to standards compliant mode causes following issues:

• [x] On the quiz page (as the used goes to next question)

  setting width/height in pixels instead of number:

  src/components/quiz/index.js#L87

  style: {
    height: this.refs.li[cqi].offsetHeight
    // should be
    height: this.refs.li[cqi].offsetHeight + 'px'
  }

• [x] On the reading page

  set height of slider content explicit in 100vh instead of 100%

• [x] logo needs a margin

  

• [x] slider image aren't centered