[1.11.x] Fixed #27998, #28543 -- Restored logging of ManyToManyField changes in admin's object history.

And prevented ManyToManyField initial data in model forms from being affected
by subsequent model changes.

Regression in 56a55566a791a11420fe96f745b7489e756fc931.

Partial backport of e5bd585c6e and
15b465c584 from master

tests/admin_views/models.py

@@ -611,6 +611,13 @@ class Pizza(models.Model):
     toppings = models.ManyToManyField('Topping', related_name='pizzas')
 
 
+# Pizza's ModelAdmin has readonly_fields = ['toppings'].
+# toppings is editable for this model's admin.
+class ReadablePizza(Pizza):
+    class Meta:
+        proxy = True
+
+
 class Album(models.Model):
     owner = models.ForeignKey(User, models.SET_NULL, null=True, blank=True)
     title = models.CharField(max_length=30)