Add fields: issueTracker and issue

[nichtich]

Add field issueTracker from https://codemeta.github.io/terms/ and/or https://w3id.org/okn/o/sd#issueTracker from Software Description Ontology to Registry, ConceptScheme, and Concordance.

Not sure about an additional field issue for individual mappings, concepts etc. as subjectOf (or an additional fuzzy field seeAlso) can already be used for the same purpose. On the other hand, linking to specific issues where an item (as conceptual entity) is discussed. The difference is more clear for Concepts:

• The concept "Apple" is the subjectOf a book about apples.
• The management of the concept "Apple" is subject of an issue about the concept "Apple".

[nichtich]

Looking for an RDF property for issue I found several unmaintained RDF ontologies. In particular

bom:Issue = http://www.ifi.unizh.ch/ddis/evoont/2008/02/bom#Issuehttp://www.ifi.unizh.ch/ddis/evoont/2008/02/bom#Issue

• subclass of sioc:Thread http://rdfs.org/sioc/ns#Thread
• subclass of Task from Issue Tracking Ontology

I'd vote for wf:task just because Issue Tracking Ontology because it has been created by TBL.