Focus variable for dropzone

app/assets/stylesheets/framework/variables.scss

@@ -150,6 +150,7 @@ $light-grey-header: #faf9f9;
  */
 $gl-primary: $blue-normal;
 $gl-success: $green-normal;
+$gl-success-focus: rgba($gl-success, .4)
 $gl-info: $blue-normal;
 $gl-warning: $orange-normal;
 $gl-danger: $red-normal;

app/assets/stylesheets/pages/note_form.scss

@@ -76,7 +76,7 @@
     &.is-dropzone-hover {
       border-color: $gl-success;
       box-shadow: 0 0 2px $black-transparent,
-                  0 0 4px rgba($gl-success, .4);
+                  0 0 4px $gl-success-focus;
 
       .comment-toolbar,
       .nav-links {