Address deprecation of spaceless tag

Since Twig 2.7, the spaceless tag is deprecated in favor of the
spaceless filter, or the apply tag with a spaceless argument. I picked
the latter so as to still benefit from syntax highlighting.

Author: greg0ire

packages/guides/resources/template/html/body/menu/content-menu.html.twig

@@ -1,5 +1,5 @@
-{% spaceless %}
+{% apply spaceless %}
     <div class="contents">
         {% include "body/menu/menu-level.html.twig" %}
     </div>
-{% endspaceless %}
+{% endapply %}