Ensure that If I have SSSS set up, and I legacy verify someone else, then I would migrate that legacy verification to signing the user, and therefore I would see that legacy verification on all of my devices. The assumption here is that the other user never enables cross-signing (so #12619 wouldn't help us here), but we still want to have the benefits of propagating trust to all of our own devices as provided by cross-signing and SSSS.

This is something the spec currently does not provide a way of doing (see discussion), and we're deferring this until the need arises.