Skip self-signing if device already signed
When adding a device to your account, both devices attempt to sign each other
using the self-signing key, but in reality only the new device needs to be
signed. This avoids a case where web would fail verification while attempting to
sign the old device because of missing private keys (since they aren't in 4S and
it hasn't requested them from the other device yet), even though signing the old
device is redundant anyway.
Part of https://github.com/vector-im/element-web/issues/14970
