Make a base user class to inherit from
Make a base user class to inherit from

Rather than every class having to:
- inherit from both `JSONModel` and `SortByStringAttributeMixin`
- define `__sort_attribute__`

…we can make a base user class which they can inherit from.

alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin