updating mobile menu timing function to use global transition variable rather than cubic-bezier

opentech/static_src/src/sass/layout/_header.scss

@@ -72,7 +72,7 @@
             background: $color--dark-grey;
             opacity: 0;
             transform: translate3d(0, -100%, 0);
-            transition: opacity, transform, 250ms cubic-bezier(0.65, 0.05, 0.36, 1);
+            transition: opacity, transform, $transition;
 
             &.is-visible {
                 opacity: 1;