Generate m.room.aliases event when the HS creates a room alias

synapse/api/auth.py

synapse/api/events/room.py

synapse/app/homeserver.py

synapse/handlers/_base.py

synapse/handlers/directory.py

synapse/handlers/message.py

synapse/handlers/room.py

synapse/rest/directory.py

synapse/storage/directory.py

synapse/storage/schema/delta/v3.sql