This is apparently less complex, and stops flake8 warning. Go automated code analysis.
Shift dexterity-related migrations to their own module to avoid trying to import archetypes-based things
Don't fall over if _relatedItemsOrder wasn't set
Merge branch 'master' of git://github.com/lentinj/plone.app.contenttypes into lentinj-master

Conflicts:
	plone/app/contenttypes/upgrades.py
Remove unused import.