[META] Improve the project screenshots functionality in the project browser

So excited to finally have screenshots in the Project Browser!!...

This is a follow-up issue to #3040, with some ideas for improvement:

1. Consider hiding the descriptions for themes on mobile, and showing just the "details" link (or alternatively showing less text compared to desktop/tablet).

2. (implies support for multiple screenshots, see https://github.com/backdrop/backdrop-issues/issues/1517) Consider adding some colorbox functionality when people click on the screenshot thumbnails, so that a larger version of the screenshot can be viewed. On mobile, instead of colorbox, we should enable some sort of thumb-enabled slider, so that people can flick through the screenshot thumbnails.

3. ~~Placeholder/default thumbnails for projects that do not provide screenshots. This would allow for consistency in the width that the description text for projects occupies.~~ Skratch, we seem to already have this :)

4. Enable screenshots/thumbnails for modules too. They do have that in d.org:

   

   This would allow adding screenshots that explain the functionality a module adds (for example before/after screenshots of the admin UI), which often times convey the functionality better/faster than words do. Example:

   

...up for discussion.