fix(client): add missing session package for Bert.alert()

themeteorchef:bert's Bert.alert() calls Session.set('bertAlert', ...)
internally, but the session package was never declared in
.meteor/packages — only reactive-var/tracker were. Every Bert.alert()
call (subscribe/unsubscribe success and error paths, ViewSubscription,
SubscriptionEditor) threw "ReferenceError: Session is not defined",
which crashed the DDP method result callback and silently ate whatever
UI update was supposed to happen next.

.meteor/versions

@@ -98,6 +98,7 @@ retry@1.1.1
 reywood:publish-composite@1.6.0
 routepolicy@1.1.2
 service-configuration@1.3.5
+session@1.2.3
 sha@1.0.10
 shell-server@0.6.1
 simple:json-routes@3.0.0