refactor(itam): Update all references to User to use get_user_model()

ref: #756 #755

app/itam/models/device_models.py

@@ -1,4 +1,3 @@
-from django.contrib.auth.models import User
 from django.db import models
 
 from itam.models.device_common import DeviceCommonFieldsName
@@ -10,6 +9,8 @@ from core.models.manufacturer import Manufacturer
 
 from settings.models.app_settings import AppSettings
 
+
+
 class DeviceModel(DeviceCommonFieldsName, SaveHistory):