Sign in form example can go offscreen if browser viewport is small

Created by: ngyikp

https://getbootstrap.com/docs/4.0/examples/sign-in/

If your viewport's height is small, the top part of the log in form can go offscreen:

Looks nastier on mobile with a horizontal keyboard active:

Fix is to set height: 100% on html and min-height: 100%; on body, removing height property from body