post votes: fix exception when voting on posts using API.

Fix an `undefined method post_vote_url` exception when doing
`POST https://danbooru.donmai.us/posts/1/votes.json`.

Also add the following API endpoints:

* https://danbooru.donmai.us/post_votes/:id.json
* https://danbooru.donmai.us/comment_votes/:id.json
* https://danbooru.donmai.us/forum_post_votes/:id.json

where `:id` is the vote ID, not the post ID.

app/controllers/post_votes_controller.rb

@@ -8,6 +8,11 @@ class PostVotesController < ApplicationController
     respond_with(@post_votes)
   end
 
+  def show
+    @post_vote = authorize PostVote.find(params[:id])
+    respond_with(@post_vote)
+  end
+
   def create
     @post = Post.find(params[:post_id])