Completely rewrote sliders.

They now closely follow the Material Design spec, and work in Chrome, Safari, Firefox and IE>=10.

And yes, they're still just styled `<input type="range">` elements. Well, some `<div>`s get dynamically inserted around them, but hey, you wouldn't judge if you've seen the things I've seen... My dreams were beautiful once, now only CSS remains.

Changes include:
- Implementing the whole thing in IE.
- Adding true transparency.
- Switching to pixel measurements.
- Using "transform: scale" instead of changing width/height, for performance and layout reasons.
- For the track coloring, switching away from imprecise and slow linear gradient backgrounds to a flexbox implementation.
- A ton of tiny tweaks for various visual glitches.