Add attachment deduplication tool

[benanhalt]

Need a tool for finding duplicate attachments and merging them (either automatic detection of duplicates or a manual feature of selecting two attachments and clicking "merge")

[maxpatiiuk]

Let attachment picket idea be tracked by #2999, and reword this to be for finding duplicate attachments only

[grantfitzsimmons]

It would be very nice to have a tool that allowed you to link an existing attachment from another record with a new record.

[specifysoftware]

This issue has been mentioned on Specify Community Forum. There might be relevant details there:

https://discourse.specifysoftware.org/t/open-discussion-on-associated-media-management-in-specify-7/2156/3

[grantfitzsimmons]

Need to make sure users can select an existing image