interactives-metadata.js : add category, subcategory, screenshot.

An interactive author can specify a category and subcategory for an interactive so that they can be automatically categorized on Concord's STEM Resource Finder site.

An interactive author can specify a screenshot for an interactive to show up in the Concord STEM resource finder.

[#61222898] [#61222800]

https://www.pivotaltracker.com/story/show/61222800
https://www.pivotaltracker.com/story/show/61222898