Putting this logic into the omniauth strategy is much cleaner because:
- we no longer need a Rails controller and the associated `custom_url` parameter
- we can re-use the `authn_request` instance which is automatically generated by the omniauth strategy, rather than re-implementing that logic
- the behavior is decided at runtime, rather than during initialization. This makes it testable, and is another step on the way to making the plugin multisite-compatible

This commit also introduces a spec for the feature.