_event.atom.builder

Previously we'd use "event.author_email" which translates to
"event.author.email". This would result in an extra query being executed
_for every event_ just to get the same author's Email address.

Instead of doing all this useless work we can just pass the User object
returned by "event.author" since:

1. This allows us to re-use the user object's Email address.
2. Authors are eager-loaded, so this doesn't cause any N+1 queries.