Download: Move extra channel metadata import to background

[dbnicholson]

Like the unavailable content thumbnails, move the download of the extra channel metadata to the background instead of blocking the initial starter pack download on it.

I'm marking this as a draft for now since we need some frontend work so it can be notified when new channels appear.

Helps: #592

[dbnicholson]

As discussed on Slack, this isn't going to work. The way this PR works is that the extra channel and thumbnail tasks are all created at once and put into a background queue. However, the thumbnail tasks can't be created until the channel metadata exists. There are 3 ways I can think to handle this:

1. Leave the extra channel download in the foreground. I.e., do nothing.
2. Download all the channels in the foreground immediately. This is the same as 1 except the channel metadata downloads aren't split into 2 stages.
3. Have the frontend keep driving the stages even after it's left the download page. This would kinda weird since every page would need to keep polling the download manager.
4. Have the backend drive the background stages by using a StorageHook that's notified when tasks complete. That would allow it to do something similar to how the foreground tasks are handled. I.e., a background_tasks_pending list that needs to be exhausted before moving on to the next background stage. Or you could go all the way and use this hook to drive all the stages and then the frontend polling would purely return status without also trying to manage the state machine.

[dbnicholson]

I'm closing this since it's not going to ever work this way.