This supports Discord as an auth provider. It depends on the next
release of the Arcade Engine to work, so ~we'll hold off on merging for
now.~ we'll do it live!
Fixes a circular import issue where `arcade.sdk -> arcade.core` but also
`arcade.core -> arcade.sdk`. My mistake!
Moved some of the shared classes down into `core`, and re-exported them
to `sdk` to keep the expected interface for devs.
