Add system check for SINGLE_USER mode

users/__init__.py

@@ -0,0 +1,16 @@
+from django.core.checks import Warning, register
+
+
+@register("users")
+def check_single_user_implies_disabled_registration(app_configs, **kwargs):
+    from bugsink.app_settings import get_settings
+    errors = []
+    if get_settings().SINGLE_USER and get_settings().USER_REGISTRATION != "CB_NOBODY":
+        errors.append(
+            Warning(
+                "You're in SINGLE_USER mode, but USER_REGISTRATION is not set to 'CB_NOBODY'. This means that users "
+                "can still register, which is probably not what you want.",
+                id="users.W001",
+            )
+        )
+    return errors