Style case-study page

Add some styling for a case study show page.

- Setup IE specific stylesheets
- Improve the slimmer setup
- Add presenter to interpret content store data
- Use components to build show page
- Add minimal styling for things not covered by the show page
No, not 'static' - that's not exposed externally.   This will do the right thing, although it could use the [abstraction in Plek](https://github.com/alphagov/plek/blob/master/lib/plek.rb#L33-L35) `Plek.current.asset_root`.