Update JS Bin template in CONTRIBUTING.md

Refs: #14109

Link: http://jsbin.com/zazif/1/edit?html,output

That bin still refers to https://github.com/necolas/issue-guidelines for bug reporting guidelines, is that still valid?

Also, I switched to using a CDN for the Bootstrap files, as @cvrebert mentioned a couple of times that we don't endorse hotlinking to getbootstrap.com
The reason why I chose jsDelivr over BootstrapCDN is that they actually have a up-to-date `/latest/` version. (See [jsDelivr](http://cdn.jsdelivr.net/bootstrap/latest/css/bootstrap.min.css) and [BootstrapCDN](http://maxcdn.bootstrapcdn.com/bootstrap/latest/css/bootstrap.min.css).)

/cc @cvrebert