Delete unpublished attachments

[leplatrem]

Scenario is this:

• I want to add an attachment
• I browse for files and upload one
• it gets stored in the main-workspace folder under a randomly chosen name and added to the record
• I decide that I was wrong, and delete the record
• the uploaded file remains in there, publicly accessible for anyone luckily enough to guess the uuid (😬)

When we approve changes for a collection, we could look at all uploaded files that were uploaded since last approval and delete all that are not referenced by any published record.