Silently reset cross-signing on new login if there's no other devices, no 4S, and no encrypted chats

Right now, if someone only ever uses Element for unencrypted chats, and logs out of their only device without setting up 4S, we unnecessarily present them with a scary "reset your keys" dialog when they login again. I think it's probably worth confirming the reset if they do have encrypted chats, so they're aware they'll need to reverify with people (especially in a TOFU world), but if there aren't any encrypted chats, we shouldn't bother prompting the user about the reset.