Full layout refactor

`mdl-layout` is a very complex component, with many moving parts and different options to it. It should be simplified by:
- Splitting it into multiple, smaller components (`mdl-header`, `mdl-drawer`, etc.) that are easy to get to work together (by tying them with the `data-mdl-*` properties, for example).
- Remove as much mandatory JS behaviour as possible; look into how much we can do with just CSS (some things, like waterfall headers, will probably always need it, though).