[1.7.x] Improve error message for missing migrations in an app
[1.7.x] Fixed #22563: Added migration to admin, fixed a few more loader issues.
[1.7.x] Fixed #22660: Doc'd you can't have unmigrated apps depend on migrated
[1.7.x] Fixed #22833: Autodetector not doing through mapping correctly
[1.7.x] Persist non-schema-relevant Meta changes in migrations
[1.7.x] Fixed #22470: Full migration support for order_with_respect_to
[1.7.x] Fixed #22568: Better proxy model support in migrations