Add human-readable cron descriptor

[mxdvl]

https://github.com/guardian/amigo/blob/c4fd08017ecf08e0e6a39ea540ed9b5cf744deb2/app/views/showRecipe.scala.html#L61

It would be handy to get a human-readable description next to the cron jobs, to prevent users from having to click away.

The cron-expression-descriptor package seems to exist in many languages, including JavaScript if we want to to this client-side only.