stages/invitation: move invite signals from core to app

passbook/core/signals.py

@@ -2,6 +2,4 @@
 from django.core.signals import Signal
 
 user_signed_up = Signal(providing_args=["request", "user"])
-invitation_created = Signal(providing_args=["request", "invitation"])
-invitation_used = Signal(providing_args=["request", "invitation", "user"])
 password_changed = Signal(providing_args=["user", "password"])