New top bar

This implements the new style top bar which allows a navigation to be embedded into it.

Large changes which are worth a note:

- Renamed the `media` mixin to `media-down` this is to stop it from clashing with the media query mixin from the toolkit. This will let you use both mixins and stops one from accidentally overwriting the other based on order of inclusion (cc: @phae)
- Add template for slimmer to use to wrap a proposition navigation which will be inserted into the bar

I will open a pull request against slimmer to let apps take advantage of being able to inject a navigation element.