Allow users to delete a file from backup using rdiff-backup-delete

• Add a button to allow users to delete a file or folder with all its history from the backup
• Prompt users for confirmation before performing the deletion operation
• Deletion operation should run in background
• Deletion operation will launch a rdiff-backup-delete process to perform the deletion
• Files or folders marked for deletions should be identified visually using an icon or a colour.
• Make deletions only available for Maintainer and Admin roles