Refactor slices - part I

As this is going to end up being a large refactor, I'm going to incrementally check in along the way, so as not to just suddenly drop a 70 file pull request on anyone. :-)

This is an initial refactor mainly to the news container, to pull the slices out into separate templates. The idea is we should be able to get to a stage where we move the yucky logic for the index offsets and template de-duping out of the templates entirely, and just compose containers from slices ... (As for how easy this is going to be in reality, I'm still not sure - it does clean up the news container logic a bit just doing this though.)

I've also changed the container templates to have absolute imports so as to be usable inside of IntelliJ.

@stephanfowler @kaelig @janua How's this look so far?