Change .html.twig to .html

Change other file extensions for consistency.

source/_partials/latest-posts.html

@@ -0,0 +1,11 @@
+{% if data.posts %}
+  <section class="latest-posts">
+    <h2>Latest Posts</h2>
+
+    <ul>
+      {% for post in data.posts|slice(0,5) %}
+        <li{% if page.url == post.url %} class="active"{% endif %}><a href="{{ post.url }}">{{ post.title }}</a></li>
+      {% endfor %}
+    </ul>
+  </section>
+{% endif %}