Add a script to find and request all image versions in all posts

[MrSunshyne]

Why?

This is useful to force ghost to create all image versions for all images, especially when the content is being consumed by a static(SSG) frontend and the frontend requires responsive images to exist.

Ghost only creates the images when they are requested. In certain use-cases, such as SSG frontends, where ghost is no longer accessible after the built frontend is deployed, missing responsive images can be a pain. Instead of manually visiting all pages, in different screen sizes, this script accesses all of them to help ghost generate the images.