We can't prevent document editors from copy/pasting content to from one
document to another. The problem is that copying content, will copy the
urls pointing to attachments but if we don't do anything, the reader of
the document to which the content is being pasted, may not be allowed to
access the attachment files from the original document.

Using the work from the previous commit, we can grant access to the readers
of the target document by extracting the attachment keys from the content and
adding themto the target document's "attachments" field. Before doing this,
we check that the current user can indeed access the attachment files extracted
from the content and that they are allowed to edit the current document.