fix(attachments): fix missing URLs for files

Attachments couldn't be downloaded because no URL was being passed from the server for file objects. This is because I created a new files resolver for the Manuscript graphql type, but I didn't run the files through getFilesWithUrl before returning them. As Vignesh pointed out, the resolver I added was a duplicate anyway, and unneeded.

I have removed the new resolver and slightly improved the old resolver to obtain files in a single DB query rather than two.