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/forum_post_votes_controller.rb

@@ -8,6 +8,11 @@ class ForumPostVotesController < ApplicationController
     respond_with(@forum_post_votes)
   end
 
+  def show
+    @forum_post_vote = authorize ForumPostVote.find(params[:id])
+    respond_with(@forum_post_vote)
+  end
+
   def create
     @forum_post = ForumPost.find(params[:forum_post_id])
     @forum_post_vote = authorize ForumPostVote.new(creator: CurrentUser.user, forum_post: @forum_post, **permitted_attributes(ForumPostVote))