Download collections in the background, JS part

[manuq]

(JS part to avoid saying ...background, frontend part)

Since #549 the collections download has a new step for downloading all thumbnails. The backend reports "completed" to the frontend when these tasks are queued (not finished) so the thumbnails download continues in the background. But the frontend doesn't have a way to know when the thumbnails are available, so the frontend doesn't refresh. We want to also move the extra metadata stage to the background #650 .

The simplest way would be to monitor the background tasks from the root component and then reload the page. But this has been noted as not very user-friendly in the past. The user might be reading or playing content when suddenly the page is reloaded. So we need a bit of design thinking to communicate when this happens and let the user confirm the page refresh.

[manuq]

I'm deassigning myself from this task. There is useful information for how to approach this in this comment: #650 (comment)