Use --end class for header nav

Uses `govuk-header__navigation--end` class which
does away with the need for a custom override.

Based on suggestion by @quis:

https://github.com/alphagov/notifications-admin/pull/3181#discussion_r361977771

app/templates/admin_template.html

@@ -124,7 +124,8 @@
   {{ govukHeader({
     "homepageUrl": url_for('main.show_accounts_or_dashboard'),
     "productName": "Notify",
-    "navigation": navigation
+    "navigation": navigation,
+    "navigationClasses": "govuk-header__navigation--end"
   }) }}
 {% endblock %}