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/policies/comment_vote_policy.rb

@@ -7,6 +7,10 @@ class CommentVotePolicy < ApplicationPolicy
     !record.is_deleted? && (record.user_id == user.id || user.is_admin?)
   end
 
+  def show?
+    can_see_votes? || record.user == user
+  end
+
   def can_see_votes?
     user.is_moderator?
   end

app/policies/post_vote_policy.rb

@@ -6,4 +6,8 @@ class PostVotePolicy < ApplicationPolicy
   def destroy?
     unbanned? && record.user == user
   end
+
+  def show?
+    user.is_admin? || record.user == user
+  end
 end