Override the menu output with the list-less approach from static layout

See https://gitlab.com/agaric/sites/agaric-com/issues/77 for a11y concerns with this approach.

agaric/templates/navigation/menu.html.twig

@@ -0,0 +1,53 @@
+{#
+/**
+ * @file
+ * Default theme implementation to display a menu.
+ *
+ * Available variables:
+ * - menu_name: The machine name of the menu.
+ * - items: A nested list of menu items. Each menu item contains:
+ *   - attributes: HTML attributes for the menu item.
+ *   - below: The menu item child items.
+ *   - title: The menu link title.
+ *   - url: The menu link url, instance of \Drupal\Core\Url
+ *   - localized_options: Menu link localized options.
+ *   - is_expanded: TRUE if the link has visible children within the current
+ *     menu tree.
+ *   - is_collapsed: TRUE if the link has children within the current menu tree
+ *     that are not currently visible.
+ *   - in_active_trail: TRUE if the link is in the active trail.
+ *
+ * @ingroup themeable
+ */
+#}
+{% import _self as menus %}
+
+{#
+  We call a macro which calls itself to render the full tree.
+  @see http://twig.sensiolabs.org/doc/tags/macro.html
+
+  NOTE: We aren't using the passed-in attributes, perhaps this
+  is also something to revisit in
+  https://gitlab.com/agaric/sites/agaric-com/issues/77
+#}
+{{ menus.menu_links(items, attributes, 0, menu_name) }}
+{% set submenu_attributes = create_attribute() %}
+{% macro menu_links(items, attributes, menu_level, menu_name) %}
+  {% import _self as menus %}
+  {% set submenu_classes = [
+    'navbar-item',
+    'navbar-dropdown',
+    'is-hoverable',
+  ] %}
+  {% if items %}
+    {% if menu_level != 0 %}
+      <div{{ submenu_attributes.addClass(submenu_classes) }}>
+    {% endif %}
+    {% for item in items %}
+        {{ link(item.title, item.url) }}
+        {% if item.below %}
+          {{ menus.menu_links(item.below, attributes, menu_level + 1) }}
+        {% endif %}
+    {% endfor %}
+  {% endif %}
+{% endmacro %}