One CDN to rule them all

[SiAdcock]

There are a couple of third party dependencies for which we are not using unpkg.com as our CDN

jquery-visible

• This plugin is not available on npm yet, so is not listed on unpkg. Drop a 👍 on List on npm? customd/jquery-visible#34

addtocalendar

• The addtocalendar library is on unpkg, but there is an issue with the latest version. The owner has actually fixed the issue but has not updated the version on npm. Keep an eye on AddToCalendar/addtocalendar#9