Implement access token expiry (#5660)

changelog.d/5660.feature

docs/sample_config.yaml

synapse/api/auth.py

synapse/api/errors.py

synapse/config/registration.py

synapse/handlers/auth.py

synapse/handlers/register.py

synapse/storage/registration.py

synapse/storage/schema/delta/55/access_token_expiry.sql

tests/api/test_auth.py

tests/handlers/test_auth.py

tests/handlers/test_register.py

tests/rest/client/v1/test_login.py

tests/storage/test_registration.py