Fix /unrelate to work with epic URLs

After fixing /relate quick action to work with epic URLs, we should also fix it for /unrelate.

The bug is caused because we don't the the references for epics in the Service and can be fixed similar to how /relate got fixed: !180188 (9e903a2b)

def references(extractor)
  super + ::WorkItem.id_in(extractor.epics.map(&:issue_id))
end