Trigger event onFeedUpdate on hub push new feed updates
Add plugin which handles onFeedUpdate event, but with no content
Add existsfeedupdatesAction to controller

It iterates about all cache files and search for files which matches the given xhub_subscription
Add resourcewithfeedAction; its an action using in our test environment
Add getSubscriptionByResourceUri to Subscription class

You will get the associated subscription id by the given resource uri
Check if there are feed updates (by looking for particular files) and show message on the right sidebar