Basic display of court title from Content Store

Followed pattern from manuals-frontend of having "feature" specs which try to
write the specs in an abstract way but without going to the difficulty and
indirection of Cucumber features. The particular assertions don't demonstrate
it very well, but this is what I'm trying to achieve:
https://github.com/alphagov/manuals-frontend/blob/09c5590c83458ff0e1e8ad96c2b50bb0437a876c/spec/features/viewing_a_manual_spec.rb
Gracefully handle requests for non-existent courts

Rather than assume that a document came back from content-store, check if it
did and if it didn't, return a 404.

When we add Slimmer, it'll get a nicely-formatted page.
Style pages to look like GOV.UK

Use Slimmer (and hence Static) to add the GOV.UK header and footer to our
pages.

Also use the title govuk_component to render the h1 in the GOV.UK style.

Use the core_layout to avoid loading the styles for many formats that this app
doesn't provide, as is the recommended style.