approvals: remove post undelete endpoint.

Remove `POST /moderator/post/undelete` endpoint. Replace it with
`POST /post_approvals` instead.

Fixes it so that undeleting a post has the same behavior as approving a
post. Namely, it reloads the page instead of just flashing a "Post was
undeleted" message.

app/controllers/moderator/post/posts_controller.rb

@@ -1,7 +1,7 @@
 module Moderator
   module Post
     class PostsController < ApplicationController
-      before_action :approver_only, :only => [:delete, :undelete, :move_favorites, :ban, :unban, :confirm_delete, :confirm_move_favorites, :confirm_ban]
+      before_action :approver_only, :only => [:delete, :move_favorites, :ban, :unban, :confirm_delete, :confirm_move_favorites, :confirm_ban]
       before_action :admin_only, :only => [:expunge]
       skip_before_action :api_check
 
@@ -19,11 +19,6 @@ module Moderator
         redirect_to(post_path(@post))
       end
 
-      def undelete
-        @post = ::Post.find(params[:id])
-        @post.approve!
-      end
-
       def confirm_move_favorites
         @post = ::Post.find(params[:id])
       end