cubicweb #1211544 Have an "external file" backup when a file is deleted [open]

Have a bfss-thrash or bfss-backup that keeps a copy of a file when it is deleted through the user interface. Having a sysadmin backup (such as backuppc or other centralized solution) is better but not always implemented when a cubicweb is deployed. Having "backup included" (with the undo possibility in the web interface) could be a nice safety net.

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