Improvements of video uploader

[LukasKalbertodt]

• Change Opencast to validate JWT before downloading body of request #370
• Add visibility selection (to control ACL) to the uploader #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.
• 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. Show placeholder on "my videos" page after uploading a video #444

Also see sub-issues.

[dagraf]

Of the features in the section "Later" is most important to the University of Bern: > Urgency = Important

• Additional metadata: Series, Thumbnail
• Check whether the files are video files before starting to upload