Merge remote-tracking branch 'origin/release-v1.27.0' into social_login_hotfixes

changelog.d/9297.feature

changelog.d/9302.bugfix

changelog.d/9310.doc

docs/sample_config.yaml

synapse/config/sso.py

synapse/handlers/federation.py

synapse/handlers/room_member.py

synapse/res/templates/sso_auth_account_details.html

synapse/res/templates/sso_auth_account_details.js

tests/handlers/test_federation.py