Add config options to disable comments and the forum.

Add options to disable comments, the forum, and autocomplete. This is
for personal boorus and potentially for safe mode. Note that disabling
the forum may cause difficulties with creating and approving BURs.

Disabling comments and the forum merely hides them from most areas,
rather than completely removing them.

app/controllers/comments_controller.rb

@@ -4,6 +4,10 @@ class CommentsController < ApplicationController
   respond_to :html, :xml, :json, :atom
   respond_to :js, only: [:new, :update, :destroy, :undelete]
 
+  before_action if: -> { request.format.html? && !Danbooru.config.comments_enabled?.to_s.truthy? } do
+    redirect_to root_path
+  end
+
   rate_limit :create, rate: 1.0/1.minute, burst: 50
 
   def index