posts: rework post deletion to use dialog box.

Rework post deletion from using a separate page to using a dialog box,
like flagging.

* Add `DELETE /posts/:id` endpoint.
* Remove `POST /moderator/post/posts/:id/delete` endpoint.

app/views/posts/destroy.js.erb

@@ -0,0 +1,5 @@
+<% if params[:commit] == "Delete" %>
+  location.reload();
+<% else %>
+  Danbooru.Utility.dialog("Delete Post", "<%= j render "posts/partials/show/delete_dialog", post: @post %>");
+<% end %>