Subscribers broadcast group

We can now treat subscribers like a default (but not necessary) broadcast group when creating new threads/posts.
To me, they are equals. Two objects for different purposes that implement the same interface. Nesting somehow feels like `Group::Subscribers` is a part of `Group`, which it's not. `Group` is a class that interfaces with the database. `Subscribers` has nothing to do with database access.

But again, I don't feel super strongly and I'm fine with whatever you decide.