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/404.html

@@ -0,0 +1,28 @@
+{% extends "index.html" %}
+{% import "post_macros.html" as post_macros %}
+
+{% block content %}
+<!-- START NAV -->
+{% block header %}
+{{ super() }}
+{% endblock header %}
+<!-- END NAV -->
+<main>
+    <section class="container">
+        <div class="columns is-desktop">
+            <div class="column is-10-desktop is-offset-1-desktop">
+                <article itemscope itemtype="http://schema.org/BlogPosting">
+                    <div class="card article">
+                        <div class="card-content">
+                            <div itemprop="articleBody" class="content article-body">
+                                <h1>404!</h1>
+                                This page could not be found, perhaps it is not available in the current language or simply does not exist.
+                            </div>
+                        </div>
+                    </div>
+                </article>
+            </div>
+        </div>
+    </section>
+</main>
+{% endblock content %}

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">