apps: Raw payload wrapping error in QR generation for Signer

When generating “sign raw payload” message (https://polkadot.js.org/apps/?rpc=wss%3A%2F%2Frpc.polkadot.io#/signing), payload is wrapped in <Bytes> / </Bytes> but not scale-encoded. Parity Signer does not support signing non-scaled payloads (as undeclared arbitrary length of payload opens potential to pass scale-encoded payloads as something else). Thus, payload should be scale-encoded on top of <Bytes> wrap.

By the way, should <Bytes> wrapping be undone before signing? It is a bit not obvious from the interface whether the wrapping is always there.

For further details see https://github.com/paritytech/parity-signer/issues/1014