Improve styling and wording of SSO redirect confirm template (#9272)

changelog.d/9272.feature

docs/sample_config.yaml

synapse/config/sso.py

synapse/handlers/auth.py

synapse/handlers/sso.py

synapse/module_api/__init__.py

synapse/res/templates/sso.css

synapse/res/templates/sso_redirect_confirm.html

tests/handlers/test_cas.py

tests/handlers/test_oidc.py

tests/handlers/test_saml.py