Move case studies routes

[georges1996]

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This PR https://github.com/alphagov/frontend/pull/4379 needs to be merged in and this will need to be rebased to fix the case studies which have translations e.g https://www.gov.uk/government/case-studies/doing-business-in-spain

What

Handle Case Studies documents in Frontend, in preparation for them being taken out of Government Frontend

Why

Ticket: https://trello.com/c/ybpXwp34/368-move-document-type-casestudy-from-government-frontend-to-frontend

How

By moving the routes and necessary code to get the pages rendering in the frontend application

Screenshots?

Frontend

Gov-Frontend

[KludgeKML]

Looking good, a few inline comments to take care of.