DescriptorUnit content

I've been thinking about how to distribute common DescriptorUnit content in a release. This has led to a series of work items.

1. Change dump to be cabinet based, excluding any deleted cabinets.

This will allow cabinets to be posted and the original deleted, a means of removing history and keeping the Ark to a reasonable size. (Though posting and deleting a cabinet are both slow operations.)

2. Enhance dump to be able to exclude selected cabinets.

By this means, a cabinet included in the release can replace an older version of the same cabinet.

3. Make the DescriptorUnit link WikiName based, rather than a hard link. And limit valid links to being either a reference to a Topic in the same cabinet or a reference to a Topic in a special DescriptorUnit cabinet.

Now common DescriptorUnit content can be included in the release. Also, when posting between cabinets, any references DescriptorUnits which are local to the original cabinet are either resolved in the new cabinet or become invalid.