Support video captions for accessibility

[qqmyers]

Deque's Axe tool flags our video previewer for not having captions. Unlike providing an alt attribute for an image, where we can use a filename/description, captions are not something that are likely to exist unless provided by the user.

It would be interesting to explore whether the ~new aux-file capabilities in Dataverse would be a good way to associate caption tracks with a video file which could then be added in the preview as described in https://dequeuniversity.com/rules/axe/4.0/video-caption.

Since this is something that involves multiple parts (having caption tracks available, using the API and/or adding a UI to add them in Dataverse, and then work to update the previewer to list aux files and add the captions to the preview), it's not something I expect will get tackled in the short term without some motivating data.