Inherit from Product line hierarchy for Weekly summary email

This is a really good new feature, but I would like to be able to manage configure at the product group level so I didn't have to go into each product and configure each user.  Lots of clicking needed if you're managing lots of products.

Product digest emails can now be controlled by the product owner. You may not always wish for the entire product team to receive weekly emails. For example, you may only want Reviewers to receive them when you are close to completing a product release. If you are a product owner, you may now control who receives weekly digests under Product -> Users.