Use a wrapper around Persona to remove its session state

This is to fix https://trello.com/c/tJ7AeBh7

The basic idea here is that we use a wrapper around Persona that removes Persona's session state. This then means that the login session is managed exclusively by the server and worker, and Persona is an email authentication mechanism only.

This avoids various issues with the Persona session ending before the server session as detailed in the trello card.