Improvements of video uploader

[LukasKalbertodt]

Kind of important

• [x] #370
• [x] #369
• [ ] Make workflow configurable in Tobira config
• [ ] Somehow associate event with user (some DCC field?)
• [ ] Already try to ping Opencast (connection check) before file is selected. In particular: the file selector in mobile devices allows you to record a video there and then. Would be annoying if you did that and then Opencast was down.

Later

• [x] #675
• [ ] Specify thumbnail
• [ ] #683
• [x] #682
• [ ] Already store event in DB, in some special way to mark it as "awaiting". But then the user can already see it in their video list before it is finished processing. https://github.com/elan-ev/tobira/issues/444
• [ ] Show preview of video files?
• [ ] Allow uploading multiple videos. But open question: what "dimension"?
  • Are multiple videos multiple tracks? Then users need to assign "presenter", "presentation", ... to them.
  • Do we only allow uploading single track videos, but multiple of them in batch?
  • Or do we allow both? Then users need to assign which video files belong to the same event, ...