Support for RTL languages

This is for review, and I know that some stuff can be improved, but I want to get feedback if this approach is digestable for us.

Introduces some new things to stack:
- flipcss from opera
- sass, will now create app.css and app.rtl.css
- @inclued rtl {} mixin for sass for those places where we have to do some manual tweaking, might be replaced with some more robust mixins/frameworks, but I believe it's good for now
- layout will print either app.css or app.rtl.css based on userDir 
- sends X-Vary header on userDir, so we can have separate CDN cache based on lang
As with the above comment, please move the changes that don't belong to the RTL feature to another PR.