Simplify CSS setup, everything is in global.min.css, loaded through a link in the head

In huge pages (for example: discussions) we have a bad perceived performance because the stylesheet is very far down the DOM, so styling kicks in very late, creating a undesirable flash of unstyled content.

Other issues: 
- Rendering quirks in Chrome (color repeating between zone backgrounds on fronts)
- Engineering confusion (not clear what should go in the head when we create a new module)
- Having some modules in the head breaks the cascade when creating an experiment
- Rules put in the head is supposed to be styling that is "above the fold" which does not make that much sense when we scale up to desktop

After much thinking we would like to bundle everything together again, "old school": One css file linked in the head of the document.

We'd like to test if this has an impact or not on load event.
If it has a very negative impact we should revert this commit.

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