Use govuk-content-schemas to run contract (integration) tests

This adds an explicit dependency on [govuk-content-schemas](https://github.com/alphagov/govuk-content-schemas), running a series of integration tests against the examples within it. This gives us more confidence that the government-frontend can render content store content without error, assuming it conforms to the schema.

Story: https://trello.com/c/2DhrVkzr/70-add-tests-to-government-fronted-to-verify-page-rendering-against-examples-in-schema