Support webp images

Via Zendesk, we received this request:

could you please add a possibility to allow displaying WebP-Images for maps and content?
Ideally this is done via a wrapping -tag to enable downwards compatibility for browsers, who don’t support WebP yet.

It's reasonable to add support for uploading webp images and supporting them in the image mediatype (see also the media type identification process) and, perhaps, in custom map markers.

However, it is unlikely that StoryMapJS will support the <picture> element, as that would then involve also supporting upload of (or automated creation of) alternative sources. Supporting user upload of (or specification via URL of) multiple sources would add far too much complexity to the user experience, and the is no evidence of a community of users who would take advantage of multiple image support outside of the hosted experience.

Start with source/js/media/types/VCO.Media.Image.js and the media type identification process in source/js/media/VCO.MediaType.js. Check how image uploads and custom map markers are handled. Done means WebP images can be uploaded and displayed for maps and content, while the issue's requested picture-element fallback remains out of scope.