Refactor the login handler
The login handler is now a simpler, standalone http.Handler wrapper. The separation between the SSO application and the login handler is now fairly complete. The login handler no longer forces the user to a specific workflow via session cookies, but it works on a request-by-request basis instead, which makes the "back" button works as expected (allowing the user to bail out of a broken 2FA process, for example). Session handling has been simplified as well: there is a single session for authentication and login state, which should remove the opportunity for session synchronization errors.
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