Allow file uploads in the background

[seathebeauty]

Sync is great for many use cases, but it's also common to want to upload one-off files/folders that may be of a larger size.

Since current large uploads fail if not in foreground with display active, it would be great to allow these large uploads to continue in the background, perhaps with a notification of their progress as well.

[ianopolous]

Yep. We can do this by the front end triggering a similar notification to the sync starting.