Add variable for default cursor

Currently our disabled elements use `cursor: not-allowed;` which on hover shows a circle with a slash through it. It's intended to show the click is, well, not allowed, but it's not something folks see all that often I feel. In addition, we don't currently have an easy way to customize this should you want to adjust it.

~~So, this PR adds the variable *and* sets it to `default` instead of `not-allowed`. This also has the added benefit of fixing a recent inconsistency in behavior—most modern browsers show no cursor at all due to `pointer-events`. This brings IE10 and below closer in line with that.~~

**Edit:** Alright, screw that, we're going with `not-allowed` still and adding the variable.

/cc @cvrebert @JasperHorn