[WIP] Bug 1036486 less2stylus

See https://bugzilla.mozilla.org/show_bug.cgi?id=1035783

[staticfiles](https://github.com/mozilla/olympia/pull/110) PR needs to be landed before.
@yohanboniface I would say this isn't simply a 'matter of taste'. Consistency between our projects is something we should aim for and a large selection of Mozilla projects use the brace syntax for Stylus.

In addition I would agree with@cvan it makes sense to keep the stylus syntax that makes it look more like the language of the compiled files as it's more likely to make contributors feel at home even if they've not used stylus before. This is not python, so I'm not clear how being "Pythonic" should factor?

As a front-end dev for me looking at this code it's definitely harder to understand and there's some loss of immediacy. I struggle to understand why you would choose to potentially add unnecessary cognitive overhead for people that come to make CSS changes if they aren't using the abstracted stylus syntax on a daily basis.