Allow multiple PWs (one time token) per user

[Tazinho]

Thank you very much for this great package. I have a use case for which we want to implement a token solution. Each token can be used exactly once. Each user can have multiple tokens. Unfortunately, shinyauthr checks that the username entered is unique in the user data. After a short look at the code in the login script I think that this can be easily rewritten. Would you consider this option? If it would help, I could also try to set up an appropriate pull-request.