Prevent notifications from being too tall and unclosable

Closes #1558

- Adds a wrapping tag around the notification message
- Change tests to look for new wrapping tag

For the fix to work properly, it needs styles from the [0.8 branch](b98029911c) of Ghost-UI

ghost/admin/templates/components/gh-notification.hbs

@@ -1,4 +1,6 @@
 <section {{bind-attr class=":js-notification message.typeClass"}}>
-    {{{message.message}}}
+    <span class="notification-message">
+        {{{message.message}}}
+    </span>
     <a class="close" {{action "closeNotification"}}><span class="hidden">Close</span></a>
 </section>