Update responsive navbar padding to use variables

Created by: wojtask9

Currently .nabvar-expand-* classes adds padding to .nav-link wrapped by media-breakpoint-up https://github.com/twbs/bootstrap/blob/v4-dev/scss/_navbar.scss#L168-L171

This should be moved few lines above to media-breakpoint-down https://github.com/twbs/bootstrap/blob/v4-dev/scss/_navbar.scss#L143

It would be great if those paddings were variables.

EDIT: should be moved to media-breakpoint-down