Add account expiration feature

changelog.d/5027.feature

docs/sample_config.yaml

synapse/api/auth.py

synapse/api/errors.py

synapse/config/registration.py

synapse/storage/prepare_database.py

synapse/storage/registration.py

synapse/storage/schema/delta/54/account_validity.sql

tests/rest/client/v2_alpha/test_register.py

tests/test_state.py