NavBar responsive breakpoints are hardcoded

[merlos]

# Example
@media only screen and (max-width: 767.98px) {

Use instead the mixins defined in:
https://getbootstrap.com/docs/4.1/layout/overview/#responsive-breakpoints

For instance:
@include media-breakpoint-down(xs) { ... }
@include media-breakpoint-down(sm) { ... }
@include media-breakpoint-down(md) { ... }
@include media-breakpoint-down(lg) { ... }

[merlos]

Reopened. At least _navbar.scss still has hardcoded breakpoints.

https://github.com/unicef/design-system/blob/master/scss/_navbar.scss

Do review all scss to double check there are no more hardcoded breakpoints.