is-modern and is-not-modern browser support classes

Until a few days ago, a browser that did not cut the mustard was put in the "js-off" bucket. Modern browsers were in the "js-on" bucket.

We are introducing a new tier of support.

### Before

- `html.js-off`: browser has JavaScript turned off / does not support JavaScript
- `html.js-on`: browser cuts the mustard

### After

- `html.js-off.is-not-modern`: browser has JavaScript turned off / does not support JavaScript
- `html.js-on.is-not-modern` JavaScript is turned on, but browser does not cut the mustard
- `html.js-on.is-modern`: browser cuts the mustard

### On x-browser testing

To test rendering in older browsers without opening IE8, Safari on iPad 1 or Opera Mini, it's no longer enough to turn off JavaScript in developer tools, and we would need to actually test on a browser that has JS on, but does not cut the mustard. This comes in the way of fun and productivity.

I think the solution is a user preference switch that forces the "is-not-modern" state. /cc @ironsidevsquincy @phamann @sndrs what do you think?

![ ](http://gifs.joelglovier.com/high-five/awkward-high-five.gif)
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