Fixed #22397 -- Issues removing M2M field with explicit through model.

Changed the migration autodetector to remove models last so that FK
and M2M fields will not be left as dangling references. Added a check
in the migration state renderer to error out in the presence of
dangling references instead of leaving them as strings. Fixed a bug
in the sqlite backend to handle the deletion of M2M fields with
"through" models properly (i.e., do nothing successfully).

Thanks to melinath for report, loic for tests and andrewgodwin and
charettes for assistance with architecture.

tests/migrations/test_state.py

@@ -291,3 +291,33 @@ class StateTests(TestCase):
         ))
         self.assertNotEqual(project_state, other_state)
         self.assertEqual(project_state == other_state, False)
+
+    def test_dangling_references_throw_error(self):
+        class Author(models.Model):
+            name = models.TextField()
+
+        class Book(models.Model):
+            author = models.ForeignKey(Author)
+
+        class Magazine(models.Model):
+            authors = models.ManyToManyField(Author)
+
+        # Make a valid ProjectState and render it
+        project_state = ProjectState()
+        project_state.add_model_state(ModelState.from_model(Author))
+        project_state.add_model_state(ModelState.from_model(Book))
+        project_state.add_model_state(ModelState.from_model(Magazine))
+        rendered_state = project_state.render()
+        self.assertEqual(len(rendered_state.get_models()), 3)
+
+        # now make an invalid one with a ForeignKey
+        project_state = ProjectState()
+        project_state.add_model_state(ModelState.from_model(Book))
+        with self.assertRaises(ValueError):
+            rendered_state = project_state.render()
+
+        # and another with ManyToManyField
+        project_state = ProjectState()
+        project_state.add_model_state(ModelState.from_model(Magazine))
+        with self.assertRaises(ValueError):
+            rendered_state = project_state.render()