PERF: ensure priority for rake rebake tasks is ultra low

Flooding the default queue with lots of jobs is never a good thing. This
reduces the risk of sidekiq going into a bad state post rebake

lib/tasks/posts.rake

@@ -111,6 +111,9 @@ def rebake_posts(opts = {})
 end
 
 def rebake_post(post, opts = {})
+  if !opts[:priority]
+    opts[:priority] = :ultra_low
+  end
   post.rebake!(opts)
 rescue => e
   puts "", "Failed to rebake (topic_id: #{post.topic_id}, post_id: #{post.id})", e, e.backtrace.join("\n")