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/comments_controller.rb

@@ -2,6 +2,8 @@ class CommentsController < ApplicationController
   respond_to :html, :xml, :json, :atom
   respond_to :js, only: [:new, :update, :destroy, :undelete]
 
+  rate_limit :create, rate: 1.0/1.minute, burst: 50
+
   def index
     params[:group_by] ||= "comment" if params[:search].present?