[UX] Allow multiple screenshots for modules themes and layouts

This is a thought I have been pondering for a while. App stores on both Apple and Android allow you to preview screenshots of your app and what it does. I really like that part of selecting an app. I would like to see something similar for Backdrop.

We currently allow screenshots for themes, but not (really) for modules or layouts. I would love to be able to handle screenshots consistently across all project types, and deal with them a little better than we do now. I would like to allow for more than one screenshot, so that modules that provide a variety of features could show them all, or themes that have many beautiful pages to display can do so. I propose integration in the following way: - any project can add a directory screenshots that can contain images. - The GitHub packaging script will remove these screenshots before generating .zip files for download - The backdropcms.org site will pull in these images, and do the following with them: 1) The first image will be used in the project listing page if a specific image (screenshot) is not provided 2) All images can be displayed in a carousel or slideshow of some type on the project page