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/helpers/_background.scss

@@ -1,5 +0,0 @@
-@if $enable-gradients {
-  @each $color, $value in $theme-colors {
-    @include bg-gradient-variant(".bg-gradient-#{$color}", $value);
-  }
-}