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/_root.scss

@@ -12,4 +12,5 @@
   // See https://github.com/sass/sass/issues/2383#issuecomment-336349172
   --bs-font-sans-serif: #{inspect($font-family-sans-serif)};
   --bs-font-monospace: #{inspect($font-family-monospace)};
+  --bs-gradient: #{$gradient};
 }