Fixed #23618 -- Allowed apps with no models to still have migrations

Basically a4737bf6ae reapplied.

Conflicts:
	docs/releases/1.8.txt
Fixed #23556 -- Raised a more meaningful error message when migrations refer to an unavailable node

Conflicts:
	django/db/migrations/graph.py
Fixed #23733 -- Fixed squashing migrations that depend on multiple apps.
Upgrade django version for oscaro

Include updates about squashing migrations from django master (with a
commit from myself), needed when squashing migrations with dependencies
into other apps

https://github.com/django/django/commit/85086c815873c4cf0bc2f1f2809119088cbe55f1

https://github.com/django/django/commit/fd061b6591ddc0b3045661ff22243919a7c76f17