Show loading notifications when things are happening

[alastair]

When we are loading the score or the annotation there is no indication that we're waiting for something to happen.
There was a "loading" div that we showed and hid, but this isn't a good way to manage the layout of the app using react.
We should look into perhaps creating some hooks that allow us to obtain a loading boolean when while items are still being loaded.

[fedezubo]

I do have a loading spinner available that is animated and looks nice. I think it's called spinner and I was hiding and showing it via queryselector. You can use that if you like :)

[alastair]

I deleted the loading spinner because we shouldn't be modifying the dom directly from react operations. This issue is a reminder to add it back in but use proper react functionality to do so.