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/dcim/forms/model_forms.py

@@ -1,5 +1,5 @@
 from django import forms
-from django.contrib.auth.models import User
+from django.contrib.auth import get_user_model
 from django.contrib.contenttypes.models import ContentType
 from django.utils.translation import gettext as _
 from timezone_field import TimeZoneFormField
@@ -236,7 +236,7 @@ class RackReservationForm(TenancyForm, NetBoxModelForm):
         help_text=_("Comma-separated list of numeric unit IDs. A range may be specified using a hyphen.")
     )
     user = forms.ModelChoiceField(
-        queryset=User.objects.order_by(
+        queryset=get_user_model().objects.order_by(
             'username'
         )
     )