Added themed 404 page

Currently requires a hacky fix of the index.html template as the
404 only has the config in its context.

templates/index_macros.html

@@ -53,6 +53,18 @@
 
 {% macro navbar() %}
 {% if config.extra.zulma_menu or config.extra.zulma_brand or config.build_search_index %}
+
+{# This is used for the 404 template, as it only gets the config in its context.
+   This is clearly hacky and should be changed eventually #}
+
+{% if not lang %}
+{% set lang = config.default_language %}
+{% endif %}
+
+{% if not current_url %}
+{% set current_url = config.base_url %}
+{% endif %}
+
 <header>
     <nav class="navbar">
         <div class="container">