Support new parent field for company-managed projects

[andrebask]

Add support for the new parent field so that it is possible to create issues linked to an epic in company-managed projects.

Creating an issue linked to an Epic in company-manged projects is currently impossible. This is because of the new Parent field introduced for these kind of projects. See also #637

[ankitpokhrel]

Could you check if setting the project type to next-gen works? #738 (comment)

[andrebask]

Yeah that works, thanks!