Refactor 2FA views into user/selfservice/session modules

tests/views/test_mfa.py

tests/views/test_selfservice.py

tests/views/test_session.py

tests/views/test_user.py

uffd/fido2_compat.py

uffd/templates/selfservice/disable_mfa.html

uffd/templates/selfservice/self.html

uffd/templates/selfservice/setup_mfa.html

uffd/templates/selfservice/setup_mfa_recovery.html

uffd/templates/selfservice/setup_mfa_totp.html

uffd/templates/session/mfa_auth.html

uffd/templates/user/show.html

uffd/views/__init__.py

uffd/views/mfa.py

uffd/views/selfservice.py

uffd/views/session.py

uffd/views/user.py