* Add tests

* Add security-related tests

My first (unpublished) attempt at fixing the issues introduced (extremely
hard-to-exploit) security vulnerabilities, addressing them in a test.

* Fix authentication failures after going halfway through a sign-in attempt

* Refactor `authenticate_with_sign_in_token` and `authenticate_with_two_factor` to make the two authentication steps more obvious

If someone tries logging in to an account and is prompted for a 2FA
code or sign-in token, even if the account's password or e-mail is
updated in the meantime, the session will show the prompt and allow
the login process to complete with a valid 2FA code or sign-in token