FIX: Change /bookmarks URL back to topic list and add bookmark poster avatars (#9759)

* Changes the /bookmarks URL to show the original "topics filtered by bookmark" list instead of redirecting to user activity bookmarks (see https://meta.discourse.org/t/domain-com-bookmarks-is-showing-domain-com-u-user-activity-bookmarks-with-reminders/149252/12)
* Add the user avatar for the user who made the post that is bookmarked

app/serializers/user_bookmark_serializer.rb

@@ -24,7 +24,9 @@ class UserBookmarkSerializer < ApplicationSerializer
              :highest_post_number,
              :bumped_at,
              :slug,
-             :username
+             :post_user_username,
+             :post_user_avatar_template,
+             :post_user_name
 
   def topic
     @topic ||= object.topic || Topic.unscoped.find(object.topic_id)
@@ -94,7 +96,19 @@ class UserBookmarkSerializer < ApplicationSerializer
     topic.slug
   end
 
-  def username
-    post.user.username
+  def post_user
+    @post_user ||= post.user
+  end
+
+  def post_user_username
+    post_user.username
+  end
+
+  def post_user_avatar_template
+    post_user.avatar_template
+  end
+
+  def post_user_name
+    post_user.name
   end
 end