signon: add an appropriate audit trail for edits from the admin site

Signed-off-by: Mattia Rizzolo <mattia@debian.org>

signon/admin.py

@@ -8,3 +8,9 @@ class IdentityAdmin(admin.ModelAdmin):
         "username",
         "person__fullname", "person__email",
     )
+
+    def save_model(self, request, obj, form, change):
+        """
+        Given a model instance save it to the database.
+        """
+        obj.save(audit_author=request.user, audit_notes="edited from admin")