Change .html.twig to .html

Change other file extensions for consistency.

source/_layouts/talk.html

@@ -0,0 +1,13 @@
+{% extends 'default' %}
+
+{% block content_wrapper %}
+  <h1>{{ page.title }}</h1>
+
+  {% block content %}{% endblock %}
+
+  {% if page.slides_embed %}
+    <div class="slides">
+      {{ page.slides_embed|raw }}
+    </div>
+  {% endif %}
+{% endblock %}