fix(app/user): use proper UserAdmin auth form

* use `django.contrib.auth.admin.UserAdmin`.
* using `admin.ModelAdmin` do not provide forms to
  properly handle `passwords`

[1]: https://docs.djangoproject.com/en/dev/topics/auth/customizing/#custom-users-and-the-built-in-auth-forms
Merge pull request #94 from theskumar/fix/user-admin

fix(app/user): use proper UserAdmin auth form