api: add rate limit to /posts.atom endpoint.

Add a rate limit of 1 request per 2 seconds to the post RSS feed
endpoint (/posts.atom).

This lets you check your feeds 30 times per minute, or 1800 times per
hour. The previous limit was 10 requests per second.

This is because there are too many bad RSS feed reader bots constantly
checking the same tags over and over again, 24 hours a day, as fast
they can.

app/controllers/posts_controller.rb

@@ -2,6 +2,8 @@ class PostsController < ApplicationController
   respond_to :html, :xml, :json, :js
   layout "sidebar"
 
+  rate_limit :index, rate: 1.0/2.seconds, burst: 50, if: -> { request.format.atom? }, key: "posts:index.atom"
+
   def index
     if params[:md5].present?
       @post = authorize Post.find_by!(md5: params[:md5])