Add utility class for xl font

This will allow elements to set contextual 1em=16px for appropriate
icon sizing (in buttons)

src/styles/mixins/utils.scss

@@ -74,6 +74,12 @@
   font-weight: 500;
 }
 
+@mixin font--xlarge {
+  @include font-base;
+  font-size: var.$font-size--subheading;
+  font-weight: 500;
+}
+
 @mixin shadow {
   box-shadow: 0 1px 1px var.$color-shadow--base;
 }

src/styles/util.scss

@@ -50,6 +50,9 @@
   @include layout.vertical-rhythm;
 }
 
+.u-font--xlarge {
+  @include utils.font--xlarge;
+}
 // Icons
 
 // These utilities establish dimensions appropriate for icon elements