Merge branch 'social_login' into develop

changelog.d/9276.feature

changelog.d/9277.feature

changelog.d/9286.feature

changelog.d/9287.feature

docs/sample_config.yaml

docs/workers.md

synapse/config/sso.py

synapse/handlers/auth.py

synapse/handlers/register.py

synapse/handlers/sso.py

synapse/res/templates/sso.css

synapse/res/templates/sso_account_deactivated.html

synapse/res/templates/sso_auth_account_details.html

synapse/res/templates/sso_auth_bad_user.html

synapse/res/templates/sso_auth_confirm.html

synapse/res/templates/sso_auth_success.html

synapse/res/templates/sso_error.html

synapse/res/templates/sso_new_user_consent.html

synapse/rest/synapse/client/__init__.py

synapse/rest/synapse/client/new_user_consent.py