Use default source for manual identities

This ensures that identity values created from values written in the
frontend by guests are given a source value signifying so.

Resolves: GREG-186

gregui/api/serializers/guest.py

 import datetime
 
+from django.conf import settings
 from django.utils.timezone import now
 from rest_framework import serializers
 from rest_framework.exceptions import ValidationError
@@ -156,6 +157,7 @@ def create_identity_or_update(
         Identity.objects.create(
             person=person,
             type=identity_type,
+            source=settings.DEFAULT_IDENTITY_SOURCE,
             value=value,
         )
     else: