Position absolute on mobile menu

Hey guys, mobile menu is not scrollable on iOS.

Im not sure if you would like to have page visible only until menu ends, if so, I can do it but there will be needed also little of js. (adding height: calc(100% - 48px) to .main-menu fixes scrolling on position fixed, but its not the greatest experience, and calc don't work on older androids)
I don't think it's needed to have `html.menu-open`. `.menu-open` alone should be good enough!