Add support for selecting an existing attachment

[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.

Originally posted by @grantfitzsimmons in #84 (comment)

At this time, attachments can only be linked to a single record from the interface in Specify, though the tables in the database are configured so that a single attachment can be linked to multiple records.

e.g. Collection Object -> Collection Object Attachment -> Attachment

Multiple Collection Object Attachment records can link to the same Attachment, so adding an interface to search existing attachments and make the join in the UI would be desirable.

Requested By: Several users (KU Invertebrate Paleontology, KU Ichthyology, University of Copenhagen, and others)

[maxpatiiuk]

This would be really nice, and not complicated on the front-end.
The back-end might be tricker though - we might have to adjust the cascade deletion rule for attachments to not delete the attachment when related record is deleted as long as attachment is still used in some record