Rewritten reftest analyzer from XHTML to HTML5

The reftest analyzer was in desperate need of some love and attention. This patch does the following:

- Rewrite the reftest analyzer from XHTML to HTML5
- Move CSS and JavaScript into their own files
- Style the reftest analyzer a bit better: clean colors and the same font as used in PDF.js
- Adhere to the style guide as much as possible (for example: using two space indents everywhere)
- Make the reftest analyzer work in Safari: fixes https://github.com/mozilla/pdf.js/pull/4318#issuecomment-35807411