update templates to inherit themselves instead of manually doing it

site/templates/blog-post.tera

@@ -1,3 +1,5 @@
+{% extends "base.tera" %}
+{% block content %}
 <div class="blog-post">
 	<h1 class="title">{{title}}</h1>
 	<span class="timestamp">published {{timestamp}}</span>
@@ -20,3 +22,4 @@
 		{% endfor %}
 	</div>
 </div>
+{% endblock content %}