`ITrade/SendOfferToSteamId` should create a new user if one doesn't exist already

[waylaidwanderer]

For a better UX, instead of returning the message "No user was found", it might be better to automatically create a new user instead.

[rannmann]

If they have an opskins user account, it will now be created on express trade when someone attempts to initiate a trade via SendOfferToSteamId