Set up secret storage on a fresh self-hosted account

We are very careful with 3rd party / "self-hosted" accounts because we don't want to clobber any existing secret storage that is already there.

But we should handle the case where a user logs in to a fresh account, and there is no existing secret storage. In that case, it's safe for us to go ahead and create the SSSS, and to set up cross-signing and encrypted key backup, just like we do on a brand new account on our servers.