Specify thumbnail in video uploader

[LukasKalbertodt]

It would be nice to immediately set a thumbnail. We could allow uploading images or picking a timestamp from the video where a screenshot is taken (client side).

We have to see about image format: what formats does Opencast accept? And well, ideally Opencast would convert them to an appropriate delivery format, but I don't think that's currently happening? Need to investigate. Also need to investigate how exactly it needs to be sent in the ingest API.

Open question: do we want a progress bar for the thumbnail upload? It is very likely that the video is way larger and adding a separate progress bar for image upload is involved, so I'd say: no. It can just be part of the "finalize" progress activity indicator (i.e. the last step).