Enhance page attachments to allow linking to files from cloud storage providers (Dropbox, Box, etc.)

Basic concept is to allow referencing (not directly attaching) files from third party cloud storage providers such as Dropbox, Box, OneDrive, etc. Given all of these providers can be setup to use OIDC, the connections and authorization should be pretty simple. On top of this, the "Attachments" macro should be updated to clearly delineate where the attachment is actually stored.