Fix Webpack build by importing variables in mixins file (#1233)

* Fix Webpack build by importing variables in mixins file

* Import initial-variables instead of variables.sass

sass/utilities/mixins.sass

@@ -1,3 +1,5 @@
+@import "initial-variables"
+
 =arrow($color)
   border: 1px solid $color
   border-right: 0