[1.6.x] Release notes for ORM changes in 1.6

Backport of 4301d6fa36 from master
[1.6.x] Propagate get_user_model exception from get_user

Fixes #21439

Backport of 3560ef04 from master.

Conflicts:
	django/contrib/auth/tests/test_auth_backends.py
[1.6.x] Fixed #21432 -- DateTimeQuery now copies tzinfo when cloning.

Thanks Enrique Martínez for the report and @bmispelon for the tests.

Backport of 17ed99f3a3eea4bd27fa34be59c3582616ed8079 from master.
Fixed wrong override_settings path introduced by 67c30426c1370f5d6c39bd73888c3902c1c5f365.
[1.6.x] Added backported fixes to 1.6.1 release notes.

Backport of 8e6d1b97921dd3ead6afd5eb39fcd2c82244e106 from master.
Fixed Python2.6 incompatibility caused by 67c30426c1370f5d6c39bd73888c3902c1c5f365.