Allow fallback to LegacyKeyGenerator when secret_key_base is not set

Let devs upgrading from older Rails take advantage of the LegacyKeyGenerator without needing a monkeypatch
to use a blank secret_key_base
Use secrets.secret_key instead of config.secret_key

The secrets config should be the canonical source of both
secrets.secret_key_base and secrets.secret_token.

If secrets.secret_token is blank, it should be set to to
config.secret_token, just like for secret_key_base.

"secrets.secret_key_base ||= config.secret_key_base"