Add duplicate checking for uploaded attachments

[joedolson]

It can be very difficult to tell whether you've already added a given document or image to your media library if it's no longer in recent items. The lack of sorting or filtering makes specific difficult to locate.

Based on a ​conversation on duplicate detection in #core-media, we think that a starting point for this is to hash the meta data for an attachment at the time of upload and store that as attachment meta.

We could then check for that hash value before uploading a new document, and prompt the user to use the old attachment if it already exists.

The specific meta data would probably need to vary depending on the type of attachment.