Bring back the integratedWindowsLogin option?

[nicosabena]

Lock v9 had an integratedWindowsLogin option which defaulted to true that allowed skipping the blue "Windows Authentication" button for Kerberos signin when set to false (code here). This option is no longer present in v10 (although it was documented).

Lock currently shows that button based on the IP address (AFAIK), but some users might be in the network and using a Windows machine (or just not connected to AD), so the "Windows Authentication" button doesn't always make sense.

Not sure if the showing/hiding could be automated somehow, but at least giving the flexibility of setting the integratedWindowsLogin option to false seems like a handy alternative.

Note that rememberLastLogin: false can be used now to skip the Windows login button, but this may stop working if this PR gets merged.