Add sass util mixin to support selector qualification

That is, qualify a current selector with a tag. This is
because the syntax to do this in SASS is horrible

src/stylesheets/helpers/_helpers-utils.scss

@@ -59,3 +59,15 @@
     @return $rem-vals;
   }
 }
+
+/// Since the current way to qualify a class from within its ruleset is quite
+/// ugly, here is a mixin providing a friendly API to do so.
+/// @see https://css-tricks.com/snippets/sass/mixin-to-qualify-a-selector/
+/// @author Hugo Giraudel
+/// @param {String} $element-selector - Element selector
+/// @since 0.5.0;
+@mixin when-is($element-selector) {
+  @at-root #{$element-selector + &} {
+    @content;
+  }
+}