Modify SIWEPanel to hit the siwe_auth endpoint instead of the create_account endpoint once SIWE message is signed.
We also modify the SIWE_AUTH_SUCCESS payload to a boolean indicating whether the signed message is valid and login/registration can continue.
NOTE: For now we will return from siweAuthResponder BEFORE login/registration... login/registration will come later at which point the payload for SIWE_AUTH_SUCCESS will look like RegisterResponse | LogInResponse or something.
Depends on D6026