Merge pull request #4481 from matrix-org/erikj/event_builder

changelog.d/4481.misc

synapse/api/auth.py

synapse/crypto/event_signing.py

synapse/events/builder.py

synapse/federation/federation_client.py

synapse/handlers/message.py

synapse/server.py

synapse/storage/event_federation.py

tests/crypto/test_event_signing.py

tests/test_visibility.py