Add custom 404 page

app/templates/404.html

@@ -0,0 +1,13 @@
+{% extends "base.html" %}
+
+{% block title %}
+	Page not found
+{% endblock %}
+
+{% block content %}
+	<h1>Page not found</h1>
+	<p>
+		That page could not be found. The link may be broken, the page may have been deleted,
+		or you may not have access to it.
+	</p>
+{% endblock %}