Add account switching

Add multi-account management, along with account flow and sample app refinements.

This changeset completes the account manager by allowing it the ability to truly persist and switch between accounts, whether or not on the same nation. Support for multiple account manager instances is also improved. 
Related changes were required on the account model and the authenticator for improved Safari integration.

The accounts view controller is now more refined. The sample app has also been updated and made more stable. Given the multi-delete button is removed, person deletion is simplified to deleting individually on the edit person screen.