Move raw paste view to frontend

Currently, the raw paste view (GET /<paste_id>/raw) is handled by the backend while the normal paste view (GET /<paste_id>) is being routed to the frontend.

It would make sense to move this as paste encryption is done on the client side and currently paste decryption is not possible for the raw view.

It would also provide a structured way of different view options which then could include #21 .