DO NOT MERGE!!! - Added document_links field to data model
Created by: RichardHobbsPA
Task: https://hdruk.atlassian.net/browse/IG-818
Description: Added document_links field to data model for papers. This field will comprise of the new pdf, html and doi fields to match the data that Tony has already loaded into Production DB. (See IG-697) I have written a new function to validate that the doi link provided is valid.