Update schema to use class reference instead of string

Author: daneah

django-vue-graphql/backend/blog/schema.py

@@ -1,13 +1,13 @@
 import graphene
-from django.contrib.auth.models import User
+from django.contrib.auth import get_user_model
 from graphene_django import DjangoObjectType
 
 from blog import models
 
 
 class UserType(DjangoObjectType):
     class Meta:
-        model = User
+        model = get_user_model()
 
 
 class AuthorType(DjangoObjectType):