Merge branch 'social_login' into develop

changelog.d/9262.feature

changelog.d/9271.bugfix

changelog.d/9272.feature

changelog.d/9275.feature

docs/sample_config.yaml

docs/workers.md

synapse/app/generic_worker.py

synapse/app/homeserver.py

synapse/config/_base.py

synapse/config/oidc_config.py

synapse/config/sso.py

synapse/handlers/auth.py

synapse/handlers/sso.py

synapse/http/server.py

synapse/module_api/__init__.py

synapse/res/templates/sso.css

synapse/res/templates/sso_auth_account_details.html

synapse/res/templates/sso_auth_account_details.js

synapse/res/templates/sso_redirect_confirm.html

synapse/res/username_picker/index.html