Hide Progressbar if no upload in progress (#2252)
Hide Progressbar if no upload in progress (#2252)

* Fixing an error that would prevent the ProgressBar from hiding

Fixing an error that would prevent the ProgressBar from hiding, displaying a `0` in the HTML element .uppy-ProgressBar-inner whenever no files were being uploaded. This fix sets the .uppy-ProgressBar element a class, so the user can decide when to hide it (or what to do)

* Update index.js

improved comment

* removed custom class

* Fixing an error that would prevent the ProgressBar from hiding

`state.totalProgress` is not `100`. It's `0` instead   
----------

To reproduce the error:  
```
.use(ProgressBar, {
  target: '._progress_bar_',
  fixed: false,
  hideAfterFinish: true
})
```

This will set a `div` inside the `_progress_bar_` element, and this element will be visible, showing `0` to the user.

The problem is that, currently, the bar only hides when `progress === 100`, but that is never true, as `progress` resets to `0` once the upload has been completed.
That means, it is __always__ visible, and only hidden for a brief period after an upload completes (when the `progress === 100`, briefly).

This fix allows the bar to be hidden also when the progress is `0` (after and before uploads)
transloadit/uppy,transloadit/uppy,transloadit/uppy,transloadit/uppy