Change .html.twig to .html

Change other file extensions for consistency.

source/_partials/title.html

@@ -0,0 +1,14 @@
+{% set separator = '|' %}
+{% spaceless %}<title>
+  {% if page.full_title %}
+    {{ page.full_title }}
+  {% elseif page.title and site.title %}
+    {{ page.title }} {{ separator }} {{ site.title }}
+  {% elseif site.title %}
+    {% if site.subtitle %}
+      {{ site.subtitle }} {{ separator }} {{ site.title }}
+    {% else %}
+      {{ site.title }}
+    {% endif %}
+  {% endif %}
+</title>{% endspaceless %}