BUGFIX - adjust z-index for whats new drawer

After soft launching the whats new sidebar we've become aware that on
pages with other sidebars, there is competition for which component is
highest.

Adjusting the z-index here to ensure the backdrop and drawer are higher
than the issues sidebars upon rendering.

app/assets/javascripts/whats_new/components/app.vue

@@ -90,6 +90,7 @@ export default {
       ref="drawer"
       v-gl-resize-observer="handleResize"
       class="whats-new-drawer"
+      :z-index="700"
       :open="open"
       @close="closeDrawer"
     >

app/assets/stylesheets/components/whats_new.scss

@@ -57,7 +57,7 @@
 }
 
 .whats-new-modal-backdrop {
-  z-index: 9;
+  z-index: 699;
 }
 
 .whats-new-notification-count {