Xero: Switch to OAuth 2.0a

[jsayles]

As per this post: https://devblog.xero.com/an-update-on-why-we-are-saying-goodbye-oauth-1-0a-hello-oauth-2-0-6a839230908f

We need to switch to the new Xero authentication system. The current API will be considered end of live at the end of 2020 and won't work at all after March of 2021.