PERF: move crawl_topic_links to the low queue

Crawling topic links can be somewhat delayed no need to run it in the default
queue.

app/jobs/regular/crawl_topic_link.rb

@@ -9,6 +9,8 @@ require_dependency 'topic_link'
 module Jobs
   class CrawlTopicLink < Jobs::Base
 
+    sidekiq_options queue: 'low'
+
     def execute(args)
       raise Discourse::InvalidParameters.new(:topic_link_id) unless args[:topic_link_id].present?