Create and manage "sections", special issues

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(Follows OJS inspection.)

A journal may very well have standard sections. Further, perhaps not all issues include all sections. Instead of the current unique "special" issue zero, we should have a distinct "specials" class of issues. This would be a handy way to store unpublished articles by theme. They could then simply be moved into a future issue which will cover the topic.

Should we subdivide actual issues into sections? If so, sections can't be a special class of issue, but need to be a distinct concept.