Before this, users who only signed in with OAuth2 were not able to
verify their sessions with MFA because their session already used an
email factor and they couldn't use an additional email factor.
This commit changes the OAuth2 session to include 2 factors: email and
oauth2. This second special factor is used to bypass MFA checks. It is
fine to bypass MFA checks because OAuth2 is supposed to handle the
entire authentication process, verifying who the user is and we, as the
resource provider, only need to trust the OAuth2 provider.
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