Major CSS revamp

- Get rid of Bootstrap (except we still rely on some JS)
- Use BEM class names
- Rework variables/theme config
- Fix various bugs, including some on mobile

The CSS is still not ideal – it needs to be cleaned up some more. But
that can be a focus for after beta.

less/lib/mixins.less

@@ -0,0 +1,5 @@
+@import "mixins/border-radius.less";
+@import "mixins/clearfix.less";
+@import "mixins/light-contents.less";
+@import "mixins/header-background.less";
+@import "mixins/vendor-prefixes.less";