Secure checkout complete action

[lchrusciel]

This PR contains the improved implementation of #351. Current implementation will support following scenarios:

1. User is logged in - valid
2. User is not logged in and customer exists, but doesn't have an account - valid
3. User is logged in and customer exists and they are related - valid
4. User is not logged in and customer exists, but has an account - exception, he needs to log in
5. User is logged in but different mail is provided - exception

[lchrusciel]

/cc @mamazu

Will it work for you?

[mamazu]

Thanks for picking up the work.

[Zales0123]

Thanks, Łukasz! 🥇

[lchrusciel]

And Maximilian!

[Zales0123]

And Maximilian! 😄