[Backport] Use existing_fields for UserForm, and use correct User model

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 20df512edac56a97539251c6f5ede9046273255e)

Conflicts:
	oscar/apps/customer/forms.py