In the old flow, we would call the keyserver immediately after receiving a signed message from a user's wallet.

In the new flow, there are still some steps afterwards before we call the keyserver.

To make SIWEPanel support both cases, this diff:

1. Factors out the SIWE server call and related logic into a useSIWEServerCall hook
2. Add a new prop called onSuccessfulWalletSignature to SIWEPanel
3. "Lift" the SIWE server call logic in SIWEPanel up to FullscreenSIWEPanel, one of its parents
4. Use useSIWEServerCall in FullscreenSIWEPanel

Make sure the old SIWE flow still works