LPS-43915 Support notifications framework for Blogs. I have added the logic to the SubscriptionSender to make it easier to apply the framework for the rest of the core portlets (wiki, message boards, DL...)

Hey Jon,

I wanted to get your feedback about this implementation. I have created an epic to include the notifications framework in core to notify about subscriptions. 

This is the first part that sets the basic to make it easier for the rest of plugins.

Let me know if the usage of the framework is good. I think it is ;)

Thanks!