Update colours

* Remove legacy `tone-` palette values (update to latest pasteup-palette)
* Update yellow/orange in comment-related SVGs to match latest palette value
* Remove `colour()` mixin (stop abstracting the abstractions!)
* Saner colour variables (reduce amount to those actually used/needed)
@phamann I agree. What I was trying to say by it is that we are using variables for colors not just because we can't remember hex values but because we are following patterns and button/link text color (or whatever) can be a pattern. But it comes down to not use colors variables directly but using abstractions like `$c-link-primary-text` instead of using directly `$c-neutral6`. I believe that that is a little bit more modular and readable. I have no idea what neutral-6 color is but I know right away what link-primary-text looks like. White color should be treated the same. 

Anyway it is not a show stopper.