Navbar nav-link hardcoded padding

Created by: coder4life

There is general variable for padding of the .nav-links. However in navbar this padding is being hardcoded when navbar-toggleable-* is used. Specifically look at https://github.com/twbs/bootstrap/blob/v4-dev/scss/_navbar.scss#L155. No matter what variable is set for nav-links it will always be the same because of the heightened priority of CSS rules.

Proposal to fix this. Because navbar is a special type of navigation I suggest $navbar-link-padding variable to differentiate the padding from normal navs, which have different visual flow requirements.

That or copy the entire source file which seems a bit impractical. Parameters makes a lot more sense