Add a shadow-banned flag to users. (#8092)

changelog.d/8092.feature

synapse/api/auth.py

synapse/handlers/register.py

synapse/replication/http/register.py

synapse/storage/databases/main/registration.py

...e/storage/databases/main/schema/delta/58/09shadow_ban.sql

synapse/types.py

tests/storage/test_cleanup_extrems.py

tests/storage/test_event_metrics.py

tests/storage/test_roommember.py

tests/test_federation.py

tests/unittest.py