Simpler way to directly upload to Shared Albums

[vishnukvmd]

Only uploaded photos can be added to shared albums currently.

So if a user wants to adds a new photo to a shared album, we expect them to first upload that photo to one of their albums, and then add to the shared album.

Now with the introduction of Uncategorized, we can simplify this flow.

We should let users add photos to the shared albums directly, while in the background we first upload the items to their Uncategorized album, and then add a reference to it to the shared album.

Ref.

https://github.com/ente-io/photos-app/blob/6b5db665aa56596ed1fc63b5941572741b752f76/lib/services/hidden_service.dart#L46