fix(sidebar): hide sidebar instead of removing it

Previously, the sidebar was fully removed from DOM. This could cause
unwanted behavior, such as tests which test its content failing. Now, it
is just hidden. This does not affect tests any more.

src/features/sidebar/components/Sidebar.vue

@@ -61,6 +61,10 @@ const toggleSidebar = function toggleSidebar () {
 }
 
 @media (max-width: 48rem) {
+    .sidebar {
+        border: none;
+    }
+
     .sidebar:not(.expanded) .sidebar-expand-button {
         display: none;
     }