UI: "provider is not configured" during log in

With a fresh configuration folder, any attempt to login with an already existing account will result in "provider not configured" error, so that the only way of configuring the provider is to go to the "create account" pane, and add the provider there. Ideally the UX would be just entering the already existing credentials.

The API already provides methods for configuring a provider, and bonafide will accept autoconfigure=True for logging in.