Considering shift from `react-select-plus` to `react-select v2`

[moyuanhuang]

This issue is based on the assumption that we're only using react-select-plus instead of react-select. Correct me if I'm wrong, and I'll close the issue :)

The purpose of using react-select-plus was to support option groups in the dropdown, a feature that react-select didn't have.

Now that the v2 version of react-select added support this feature (I think they just back merged the code from react-select-plus), react-select-plus is not longer 'actively maintained'. We should consider changing the package from react-select-plus to react-select for the sake of compatibility. It's good also because react-select v2 now support a lot of new features that could probably enable us to do more interesting things! https://react-select.com/home