PERF: Do not include thumbnail information in default topic list payload (#10163)

Now it is only included when a theme/plugin has requested it.

app/models/topic.rb

@@ -101,12 +101,20 @@ class Topic < ActiveRecord::Base
     end
   end
 
-  def image_url
+  def image_url(enqueue_if_missing: false)
     thumbnail = topic_thumbnails.detect do |record|
       record.max_width == Topic.share_thumbnail_size[0] &&
         record.max_height == Topic.share_thumbnail_size[1]
     end
 
+    if thumbnail.nil? &&
+        image_upload &&
+        SiteSetting.create_thumbnails &&
+        enqueue_if_missing &&
+        Discourse.redis.set(thumbnail_job_redis_key([]), 1, nx: true, ex: 1.minute)
+      Jobs.enqueue(:generate_topic_thumbnails, { topic_id: id })
+    end
+
     raw_url = thumbnail&.optimized_image&.url || image_upload&.url
     UrlHelper.cook_url(raw_url, secure: image_upload&.secure?)
   end

app/serializers/listable_topic_serializer.rb

@@ -31,7 +31,7 @@ class ListableTopicSerializer < BasicTopicSerializer
   has_one :last_poster, serializer: BasicUserSerializer, embed: :objects
 
   def image_url
-    object.image_url
+    object.image_url(enqueue_if_missing: true)
   end
 
   def thumbnails
@@ -39,6 +39,10 @@ class ListableTopicSerializer < BasicTopicSerializer
     object.thumbnail_info(enqueue_if_missing: true, extra_sizes: extra_sizes)
   end
 
+  def include_thumbnails?
+    ThemeModifierHelper.new(request: scope.request).topic_thumbnail_sizes.present? || DiscoursePluginRegistry.topic_thumbnail_sizes.present?
+  end
+
   def include_unicode_title?
     object.title.match?(/:[\w\-+]+:/)
   end