Tidy up implementation of workflow-internal collections

As well as removing -PLACEHOLDER from the category name, the main change here was to allow any artifact category.

I think the artifact-id: lookup type will be unnecessary in practice, so I removed it; the generic name: lookup should be good enough.

I haven't implemented anything special for adding bare items (though it can always be done by hand using CollectionItem.objects.create). We can add those easily enough when we have an immediate use case for them.

!761 (merged) will provide the normal lookup interface for these, and #350 (closed) will handle expiration.