SignInManager:ExternalLoginSignInAsync needs flag to bypass 2fa

[brockallen]

Currently if a user's account in ASP.NET Identity is configured to require 2fa it's required for both password and external authentication. Often the external provider already does the 2fa. There should be a param on the ExternalLoginSignInAsync to bypass the 2fa step, but still keep the other checks (primarily the call to PreSignInCheck).

[HaoK]

fa976e7