Allow m2m sets to be assigned by primary key value

[Russell Keith-Magee]

At present, m2m relations must be specified using a list of object instances:

    john = Author(...)
    john.save()
    fred = Author(...)
    fred.save()
    Article.authors = [john, fred]

This proposal (and patch) would allow m2m relations be specified using primary key values in place of the object instance:

    Article.authors = [john.id, fred.id]
    # or, if you know the pk values to begin with...
    Article.authors = [4,7]

The reason for this is twofold:

1. For parity with the ability to set m2o related objects by primary key (using Article.author_id = 3)
2. To simplify deserialization, removing the need for a query to find related objects while deserializing m2m sets.

[Russell Keith-Magee]

Patch to allow m2m relations to be set by assigning primary key

[Adrian Holovaty]

(In [4448]) Fixed #3389 -- Many-to-many sets can now be assigned with primary key values