Otherwise it may cause Django start up pains, and does cause migrations
to fail. Thanks to Jonathan Moss for reporting.

(cherry picked from commit fa3f793a35)

Conflicts:
	tests/integration/customer/test_custom_user_model.py

Otherwise it may cause Django start up pains, and does cause migrations
to fail. Thanks to Jonathan Moss for reporting.

I messed up when moving Alex Moon's code to oscar.core.compat and used
the wrong User model, defeating the point of the exercise.

This is fixed now, and the filter is also used for the UserForm that
gets used in the account section.

Fixes #1283.
Fixes #1282.

(cherry picked from commit 20df512eda)

Conflicts:
	oscar/apps/customer/forms.py

I messed up when moving Alex Moon's code to oscar.core.compat and used
the wrong User model, defeating the point of the exercise.

This is fixed now, and the filter is also used for the UserForm that
gets used in the account section.

Fixes #1283.
Fixes #1282.

We now use the related manager to access the model class rather than
importing at runtime which causes a circular reference.

Fixes issues reported in comments of #1127