Add UI for localStorage-based config

It would be pretty neat to let users configure their .org sites and authentication from https://developer.wordpress.com/docs/api/console/. We shouldn't persist this data to the server though, at least not without an explicit action from the user.

Somewhat related to #39 - in order to make that work, we would at least need to store an app ID belonging to the user.

Start by reviewing the existing React/Redux UI and configuration flow, then read related issue #39 for its app-ID context. The change is complete when users can configure .org sites and authentication locally, with no server persistence unless they explicitly request it.

Written by the indexing model from the issue text.