Is confusing since "source" already captures the origin of the notifications. It's
needed to interpret things at rendering time, so we can query things as needed there.
Instead of delegating. With a polymorphic relationship, relying on a certain
attribute implies doing things like aliasing "mentioner" to "creator" or similar.
Previously when an item was `created`, we'd track the event, update the
summary text, and broadcast the notifications. But now that we have a
draft state, we shold do all of this when it's published instead.
Jorge Manrubia