Add REST API for Web Push Notifications subscriptions (#7445)

- POST /api/v1/push/subscription
- PUT /api/v1/push/subscription
- DELETE /api/v1/push/subscription
- New OAuth scope: "push" (required for the above methods)

app/serializers/web/notification_serializer.rb

@@ -54,7 +54,7 @@ class Web::NotificationSerializer < ActiveModel::Serializer
 
     def access_token
       return if actions.empty?
-      current_push_subscription.access_token
+      current_push_subscription.associated_access_token
     end
 
     def message