12794 change User ref to get_user_model (#12905)

* 12794 change User ref to get_user_model

* 12794 call get_user_model once in tests

* 12794 call get_user_model once in tests

* 12794 use settings.AUTH_USER_MODEL for FK reference

netbox/core/models/jobs.py

@@ -1,7 +1,7 @@
 import uuid
 
 import django_rq
-from django.contrib.auth.models import User
+from django.conf import settings
 from django.contrib.contenttypes.fields import GenericForeignKey
 from django.contrib.contenttypes.models import ContentType
 from django.core.validators import MinValueValidator
@@ -69,7 +69,7 @@ class Job(models.Model):
         blank=True
     )
     user = models.ForeignKey(
-        to=User,
+        to=settings.AUTH_USER_MODEL,
         on_delete=models.SET_NULL,
         related_name='+',
         blank=True,