Extract user utility code into own module

This provides more room for expansion, and reduces the amount of
arbitrary code in the __init__.py file for the new package.

app/main/views/service_settings.py

@@ -62,10 +62,8 @@ from app.main.forms import (
     SetLetterBranding,
     SMSPrefixForm,
 )
-from app.utils import (
-    DELIVERED_STATUSES,
-    FAILURE_STATUSES,
-    SENDING_STATUSES,
+from app.utils import DELIVERED_STATUSES, FAILURE_STATUSES, SENDING_STATUSES
+from app.utils.user import (
     user_has_permissions,
     user_is_gov_user,
     user_is_platform_admin,