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/views/moderator/post/posts/undelete.js.erb

@@ -1,3 +0,0 @@
-$("#c-posts #delete").show();
-$("#c-posts #undelete").hide();
-Danbooru.notice("Post was undeleted");