json_hash_builder.rb

Fixed all related specs and also changed the logic to handle edge cases. This includes exporting and exporting of group labels, which will get associated with the new group (if any) or they will become normal project labels otherwise.

Found other issues to do with not being able to import all labels at once in the beginning of the JSON - code was much simpler when we import all labels and milestones associated to a project first, then the associations will find the already created labels instead of creating them from the associations themselves.