Allow collection & items to be display by using the URN

We only support:
- https://builder.decentraland.io/collections/e95f5851-7c39-4556-be0b-b542fe77ff73
- https://builder.decentraland.io/items/a7494ef5-1e82-4e86-98fc-dd54ce79ee8f

We will need to support something like
https://builder.decentraland.io/urns/{urn} (we need to check if the urn is a collection or item) This endpoint will redirect to the specific resource.