refactor image files

* thumbnails folder: thumbnail images for tutorials (multistep decks and videos). Images should not need translations (avoid text that would need translation in the image)
* steps folder: gif images associated with tutorial steps. Images can be translated. English (default) images have the extension `.en.gif`, other language images should replace the `.en` with the corresponding locale.
* renamed some of the steps files to be more consistent and a bit less redundant
* removed a couple of files that were no longer referenced
* revised webpack config to allow localized step files to be loaded from `/static` on www