Added sidebar grow/shrink animation

refs https://github.com/TryGhost/Team/issues/1149

- beginnings of full sidebar animation, adding early to gauge impact of animation that causes layout paints for each frame

ghost/admin/app/styles/layouts/main.css

@@ -102,6 +102,7 @@
     /* background: linear-gradient(315deg,var(--whitegrey-l2),var(--white)); */
     transform: translateX(0);
     border-right: 1px solid var(--main-color-area-divider);
+    transition: flex-basis 250ms;
 }
 
 .gh-nav-menu {