Add basic frontend assets with case-studies page

- Add IE specific stylesheets
- Add them to the pre-compile list in `config/initalizers/assets.rb`
- Add them to the application layout
Set-up slimmer correctly

- Use asset path for the assets to work in production
- Use Plek to locate static for slimmer
- Enable caching on slimmer in production
Create presenter for content_item

This will need to be split at some point in the not to distant future
into a case study presenter and a generic edition like presenter. But
for the moment it is in a generic content item presenter.

Also use components to make the page closer represent the direction it
is heading in.
Add basic styling for case study page
Add format names for heading context

Add in the en localizations of the different formats and use
format_display_type to create a context for the heading.
Move translation call to the view
Use Plek abstraction rather than an ENV variable
Merge pull request #4 from alphagov/new-frontend

Style case-study page