gh: Use toml for new hub's config

- [x] Introduce toml for config
- [x] Fix cukes setup steps to use toml
- [x] Fix `authentication.feature`

@mislav  I'm not sure how much backward compatibility we need to support old hub's yaml format. But I think we have some flexibility there. We could just let our users know they need to reauthenticate for the new hub.

Changes are still in progress and I created this PR to provoke discussion before I go further