Fixed #21169 -- Reworked RelatedManager methods use default filtering

The `remove()` and `clear()` methods of the related managers created by
`ForeignKey`, `GenericForeignKey`, and `ManyToManyField` suffered from a
number of issues. Some operations ran multiple data modifying queries without
wrapping them in a transaction, and some operations didn't respect default
filtering when it was present (i.e. when the default manager on the related
model implemented a custom `get_queryset()`).

Fixing the issues introduced some backward incompatible changes:

- The implementation of `remove()` for `ForeignKey` related managers changed
  from a series of `Model.save()` calls to a single `QuerySet.update()` call.
  The change means that `pre_save` and `post_save` signals aren't called anymore.

- The `remove()` and `clear()` methods for `GenericForeignKey` related
  managers now perform bulk delete so `Model.delete()` isn't called anymore.

- The `remove()` and `clear()` methods for `ManyToManyField` related
  managers perform nested queries when filtering is involved, which may
  or may not be an issue depending on the database and the data itself.

Refs. #3871, #21174.

Thanks Anssi Kääriäinen and Tim Graham for the reviews.

tests/custom_managers/models.py

@@ -101,6 +101,22 @@ class Person(models.Model):
         return "%s %s" % (self.first_name, self.last_name)
 
 
+@python_2_unicode_compatible
+class FunPerson(models.Model):
+    first_name = models.CharField(max_length=30)
+    last_name = models.CharField(max_length=30)
+    fun = models.BooleanField(default=True)
+
+    favorite_book = models.ForeignKey('Book', null=True, related_name='fun_people_favorite_books')
+    favorite_thing_type = models.ForeignKey('contenttypes.ContentType', null=True)
+    favorite_thing_id = models.IntegerField(null=True)
+    favorite_thing = generic.GenericForeignKey('favorite_thing_type', 'favorite_thing_id')
+
+    objects = FunPeopleManager()
+
+    def __str__(self):
+        return "%s %s" % (self.first_name, self.last_name)
+
 @python_2_unicode_compatible
 class Book(models.Model):
     title = models.CharField(max_length=50)
@@ -108,10 +124,14 @@ class Book(models.Model):
     is_published = models.BooleanField(default=False)
     published_objects = PublishedBookManager()
     authors = models.ManyToManyField(Person, related_name='books')
+    fun_authors = models.ManyToManyField(FunPerson, related_name='books')
 
     favorite_things = generic.GenericRelation(Person,
         content_type_field='favorite_thing_type', object_id_field='favorite_thing_id')
 
+    fun_people_favorite_things = generic.GenericRelation(FunPerson,
+        content_type_field='favorite_thing_type', object_id_field='favorite_thing_id')
+
     def __str__(self):
         return self.title