Add Linked Elter Document functionality

We want to keep the eLTER catalogue to the one DocumentType, ElterDocument, but also want multiple options in the create dropdown, including the option to create a linked metadata document.

Rather than rigging up bespoke adjustments to the HTML framework, we add a placeholder DocumentType which exists to accept a URL and other parameters, such as a record type. Once submitted, these are used to retrieve the metadata from elsewhere and save it as a regular ElterDocument.