approvals: move post approval endpoint to /post_approvals.

Move the post approval endpoint from `POST /moderator/post/approval` to
`POST /post_approvals`.

app/views/post_approvals/create.js.erb

@@ -0,0 +1,11 @@
+<% if @approval.errors.any? %>
+  Danbooru.error("Error: " + <%= @approval.errors.full_messages.join("; ").to_json.html_safe %>);
+<% else %>
+  if ($("#c-posts #a-show").length) {
+    location.reload();
+  } else if ($("#c-moderator-post-queues").length) {
+    $("#c-moderator-post-queues #post-<%= @approval.post.id %>").hide();
+    $(window).trigger("danbooru:modqueue_increment_processed");
+    Danbooru.notice("Post was approved");
+  }
+<% end %>