Reuse authors when rendering event Atom feeds

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.

app/views/events/_event.atom.builder

@@ -5,7 +5,12 @@ xml.entry do
   xml.link    href: event_feed_url(event)
   xml.title   truncate(event_feed_title(event), length: 80)
   xml.updated event.updated_at.xmlschema
-  xml.media   :thumbnail, width: "40", height: "40", url: image_url(avatar_icon(event.author_email))
+
+  # We're deliberately re-using "event.author" here since this data is
+  # eager-loaded. This allows us to re-use the user object's Email address,
+  # instead of having to run additional queries to figure out what Email to use
+  # for the avatar.
+  xml.media   :thumbnail, width: "40", height: "40", url: image_url(avatar_icon(event.author))
 
   xml.author do
     xml.username event.author_username

changelogs/unreleased/events-atom-feed-author-query.yml

@@ -0,0 +1,5 @@
+---
+title: Reuse authors when rendering event Atom feeds
+merge_request:
+author:
+type: performance