[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.
OK, let's discuss this more :)

I see two arguments discussed here:

1. "consistency between Mozilla projects"

 This is clearly a valid argument. And this is why *we* decided to switch from Less to Stylus.
But for me, the main idea here is to keep the *work environment* consistent, so one don't need to work with Stylus on one project, and then need to install Less for another.
BTW, AMO is using both Less AND Stylus at the moment, and so the goal is to make this simpler.
This is the real starting point of this work.
But as much I see advantages in making the work environment consistent, I see some drawbacks pushing this too much. Mozilla is huge, huge. Many projects, many contributors, many decisions, many products. If we would try to keep all those projects totally synchronized  we would kill them all. Who would decide to switch off from Stylus at some point because there is a new sexy technology? We would decide to remove the braces because at some point this become the trending way of writing CSS? How do you coordinate all projects when one wants to make a change for good?
Just have a look at how many people seems interested to clean the [django-moz-header repository](https://bugzilla.mozilla.org/show_bug.cgi?id=1035783).
Let's be agile. We want to be *agile*. We want to remove constraints from our workflow, not to add some, for the good of the projects we maintain, and thus for the good of Mozilla.
So, while I totally agree with the benefits of keeping the used technology as much similar as possible between Moz projects, I think trying to synchronize the usage between all the teams and projects is pushing the button a bit too much in an unrealistic path.

2. "Reduce overhead for new contributors"

 Again, that's a valid argument, and I'm not arguing against that. But at this point, why Stylus? Why not vanilla CSS? I would be the first to sign for this move.
Who decides that using mixins and custom functions is good, but omitting brace is bad? Why? Who decides that nested rules are on the good side of the line, but removing semi-colons are not? What if we want to discuss this usage? Should we try to convince all the Moz contributors?
It's all about tracing a line. And this why I think this at the end is a discussion about tastes.
If we really want to reduce overhead, let's switch off from preprocessed CSS. Having to deal with node to install a dependency when installing the project is 80% (official stats from my head ;) ) of the overhead.

We agree on the core points: moving to Stylus is a good move for consistency, and using a CSS preprocessor is a big overhead for new contributors.

But we, current AMO maintainers, decided that omitting braces and semi-colons was on the good side of the line. You think it's not. That's what we are discussing ;) An so this is why I think that the bit we are discussing here *is* a matter of taste. And I'm much more concerned about the agility we have to push forward a project like AMO, that needs a huge amount of love, than about the question of the braces or not the braces. :)