Improve livestream fallback when a video is missing

[rossng]

Is your feature request related to a problem? Please describe.

Conference may have lots of pre-recorded videos that need to be uploaded. Occasionally, not all of them make it! Right now, Clowdr's UI doesn't indicate this clearly when the video is missing at livestream-time.

Describe the solution you'd like

Since access to the video may be restricted by element security, it's not possible for audience members to retrieve the 'missing' status of a video through GraphQL. Instead, this information needs to be embedded into the video stream somehow.

A nice solution seems to be to always fall back to the filler video for a missing video. Then add a banner using the HTML5 overlay feature of MediaLive.

Describe alternatives you've considered

We could also have a global 'item missing' video that gets inserted instead of an overlay banner. But this is less flexible IMO.

Additional context