Add `!important` to two carefully-selected utility mixins

This is the first time in my life I've ever found what seems like a
potentially valid use case for `!important`: assuring that a utility
class that establishes rhythm based on margins is able to do its job.

Typically, our components should not be styling their own margins, but
there are some cases where we do, e.g. the initial reset of button
styles, which sets `margin: 0`. (Components should style their own
padding, but margins are the domain of containing/parent elements).

These two `!important` rules allow two utility mixins that are explicitly
concerned with margins to trump the specificity of margin-resetting
rules on descendent elements.

See https://css-tricks.com/when-using-important-is-the-right-choice/

src/styles/mixins/layout.scss

@@ -60,11 +60,16 @@
  * Establish a horizontal (margin) rhythm for this element's immediate
  * children (i.e. put equal space between children).
  *
+ * This mixin uses `!important` such that it can compete with specificity
+ * of reset rules that set some of our element's margins to 0. That allows
+ * these rules—which are applied to a parent element—to be able to assert
+ * margins, as it should be able to do.
+ *
  * @param $size [5px] - Size of horizontal margin between child elements
  */
 @mixin horizontal-rhythm($size: 5px) {
   & > * + * {
-    margin-left: $size;
+    margin-left: $size !important;
   }
 }
 
@@ -72,12 +77,17 @@
  * Establish a vertical (margin) rhythm for this element's immediate
  * children (i.e. put equal space between children).
  *
+ * This mixin uses `!important` such that it can compete with specificity
+ * of reset rules that set some of our element's margins to 0. That allows
+ * these rules—which are applied to a parent element—to be able to assert
+ * margins, as it should be able to do.
+ *
  * @param $size [var.$layout-space--medium]: Spacing size (padding)
  * @FIXME Find workaround for SvgIcon exception
  */
 @mixin vertical-rhythm($size: var.$layout-space) {
   & > * + *:not([class*='SvgIcon--inline']) {
-    margin-top: $size;
+    margin-top: $size !important;
   }
 }