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

{{cookiecutter.repo_name}}/{{cookiecutter.repo_name}}/users/admin.py

@@ -1,11 +1,12 @@
 # -*- coding: utf-8 -*-
 from django.contrib import admin
 from django.contrib.auth.forms import UserCreationForm, UserChangeForm
+from django.contrib.auth.admin import UserAdmin as AuthUserAdmin
 
 from .models import User
 
 
-class UserAdmin(admin.ModelAdmin):
+class UserAdmin(AuthUserAdmin):
     create_form_class = UserCreationForm
     update_form_class = UserChangeForm