Add foreign key constraint to `event_forward_extremities`. (#15751)

changelog.d/15751.misc

synapse/_scripts/synapse_port_db.py

synapse/storage/background_updates.py

synapse/storage/database.py

synapse/storage/databases/main/event_federation.py

synapse/storage/databases/main/events.py

...e/schema/main/delta/78/03event_extremities_constraints.py

tests/storage/test_background_update.py

tests/storage/test_event_federation.py