Django-user-management

* Use `incuna-pigeon` for notifications.

* Fix `UserChangeForm` admin form `fields` to only include fields used in `UserAdmin.fieldsets`.

* Add `delete` to `ProfileDetail` view

Notes

* When an object is referencing the user model with a foreign key it is possible
to define the behavior with `on_delete`.

see https://docs.djangoproject.com/en/1.7/ref/models/fields/django.db.models.ForeignKey.on_delete

* Raise an error when user is not active at login

* Return 400 instead of 401 when `uidb64` or `token` is expired or not valid.

* Return `AuthenticationFailed` `401` instead of `404` `NotFound` for not valid
`uidb64` and `token`