Improve gradients

- Use a semitransparent gradient from light to dark which works on any background-color
- Store the gradient as a custom property (--bs-gradient)
- Remove `.bg-gradient-*` variants in favour of `.bg-gradient` which works even when `$enable-gradients` are enabled
- Add gradients to navbar, active page links and badges when gradients are enabled

scss/_navbar.scss

@@ -24,6 +24,7 @@
   padding-right: $navbar-padding-x; // default: null
   padding-bottom: $navbar-padding-y;
   padding-left: $navbar-padding-x; // default: null
+  @include gradient-bg();
 
   // Because flex properties aren't inherited, we need to redeclare these first
   // few properties so that content nested within behave properly.