NavBar responsive breakpoints are hardcoded

```
# 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) { ... }
```