Bring in the button icon changes from PR 6151

TODO: drop when rebasing on master when 6151 is merged

client/scss/components/_button.scss

@@ -309,6 +309,12 @@
         text-transform: initial;
     }
 
+    &.button--icon {
+        .icon {
+            @include svg-icon(1.5em);
+        }
+    }
+
     @include media-breakpoint-up(sm) {
         font-size: 0.95em;
         padding: 0 1.4em;

client/scss/tools/_mixins.general.scss

@@ -79,6 +79,11 @@
     position: initial;
 }
 
+@mixin svg-icon ($size: 1.5em, $position: text-top) {
+    width: $size;
+    height: $size;
+    vertical-align: $position;
+}
 
 @mixin icon () {
     @include font-smoothing;