Implement moving of steam authenticator from another device

[Rudokhvist]

Steam mobile app allows user to move authenticator to another device, as long as they still have valid phone number. From what I've heard (haven't tested myself), trade hold period is much lower this way comparing to removing authenticator and making new one, and also it's just convenient. So I decided SteamAuth (and SDA) need this feature. PR to SDA will follow.