Content deletion features

[yflory]

• Add admin tools to delete/archive a user account from its public key
  • This will delete their drive and all the documents in their drive for which their are the only owner
• Improve error screen when accessing deleted content. The error message will now correctly tell the user how the content was removed.
  • Types of deletion
    • Deleted by an owner
    • Password change (user account or document)
    • Deleted by an admin (user account or document)
    • Deleted for inactivity (documents not stored in a user drive and inactive)
  • This will improve UX in multiple places where we couldn't know if a document was deleted or if its password had been changed.
  • When some content is destroyed, a "placeholder" file is now created instead and it will store the reason of the deletion.