Improve album creation UI

[floehopper]

From this comment:

Ugh, yes the multiple file upload is all a bit janky. (1) definately sounds like a problem we should try to fix. (2) sounds like a good improvement - making a better distinction between uploaded and non-uploaded files would be good.

I cobbled together the direct upload javascript based on the rails guide. I certainly wouldn't be against replacing it altogether with a 3rd party library if we thought it'd make things more robust. We could also consider decoupling the editing of track metadata (like titles etc) from adding/editing tracks too if that helped? So that validation errors on those fields were decoupled from the long upload times of multiple tracks.

And from this comment:

That sounds worth considering. I had a quick look around and I've come across Dropzone & Uppy. The latter is possibly interesting, because it allows uploading directly from Dropbox, Google Drive, etc to Amazon S3, although it sounds as if you need to run this server application somewhere to get that functionality. Do you know anything about either of them and/or do you know of any other libraries that might be suitable?