Use Date.now() for stats page countdown

Instead of relying on window.setInterval being reliable, use whatever
the current time is to calculate how far away the next payout is.

This should work much better with browsers that don't fire timers in the
background.
Add doge.png background

Use The_pun_fart's dogelogo3.png from the doge_asset repo on GitHub as a
background.
Tweak background image interactions

- Scale background to fit, but don't enlarge the image
- Make menu buttons opaque for visibility