Soft delete strategy

A different
deletion strategy:
instead of physically deleting stuff, we mark it as deleted.
The other ingredient of this strategy is a sweeper: a process that periodically checks which items have been deleted more than 31 days ago, and then permanently deletes those items. Until that time, admins can restore them.
The sweeper also cleans up outdated temporary files, so that the temp disk will no longer fill up.

There was also a misleading button on the edition page: Delete scene. Instead of deleting the scen file, it deleted the whole edition.

The system generates more complete logging information on critical actions such as delete, download, and upload, so that future issues can be inspected more easily.

Now that the MyWork page for admin gets crammed with sections, we provide a table
of contents on the right hand side of the page.

In production

• Pure3D runs on HuC infrastructure and its data is backed up.
• The "final" design by Bas Doppen has been applied to published editions.
• The Readme has been updated.
• There is a descritpion of the system architecture.
• There is a manual for admins and owners.