fix(css): force .navbar .dropdown-menu positioning

Created by: zalog

Demo

• Before: http://codepen.io/zalog/pen/eWLvRP
• After: http://codepen.io/zalog/pen/WjgGzM

Problems: Please follow my example in before, the problems are in the same order.

• for navbars that never collapse .navbar.navbar-expand, .dropdown-menu has position: static and push parent .navbar container down
• if .dropdown-menu it's on the edge of browser, popper will move it and seems unnatural for .navbar
• if we use only .navbar (for navbar that always collapse), .dropdown-menu act in popper style

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I tried to solve that by forcing .dropdown-menu to act like before popper, and in a mobile first abordation.

cc @Johann-S, #22628 (closed), #22630