controllers: refactor rate limits.

Refactor controllers so that endpoint rate limits are declared locally,
with the endpoint, instead of globally, in a single method in ApplicationController.

This way an endpoint's rate limit is declared in the same file as the
endpoint itself.

This is so we can add fine-grained rate limits for certain GET requests.
Before rate limits were only for non-GET requests.

app/controllers/forum_topics_controller.rb

@@ -3,6 +3,8 @@ class ForumTopicsController < ApplicationController
   respond_to :atom, only: [:index, :show]
   before_action :normalize_search, :only => :index
 
+  rate_limit :create, rate: 1.0/1.minute, burst: 50
+
   def new
     @forum_topic = authorize ForumTopic.new
     @forum_topic.original_post = ForumPost.new