Refactor authn.io and add architecture that supports multiple UIs

[dlongley]

The mediator code is quite outdated here -- and has gone through several iterations of different mutations across different JS frameworks. We need to refactor it to separate out the non-JS-specific-framework code from a general JS vanilla mediator application. This cleaner layering should make the code much more maintainable and allow for different JS frameworks with minimal view code to be used over time as needed.

Additionally, this approach should help enable authn.io to support multiple UIs that work with the same underlying storage -- allowing multiple mediator experiences that can be chosen by the relying party.

[dlongley]

This work should not be started until the next release that includes 1p window flows and Web Share API-based sharing (mostly targeting native apps).

[dlongley]

Some work has now been completed that better separates the mediator code into vanilla JS vs. UI-framework-specific code, but more work is required to enable multiple UIs.