Fix snackbar notification stacking order (#2935)

app/javascript/mastodon/selectors/index.js

@@ -56,7 +56,10 @@ export const getAlerts = createSelector([getAlertsBase], (base) => {
       message: item.get('message'),
       title: item.get('title'),
       key: item.get('key'),
-      dismissAfter: 5000
+      dismissAfter: 5000,
+      barStyle: {
+        zIndex: 200
+      }
     });
   });