rename .html templates to .jinja2 for more consistency

templates/badrequest.jinja2

@@ -0,0 +1,9 @@
+{% extends "base.html" %}
+
+{% block title %}Bad Request - {{ app_name }}{% endblock %}
+
+{% block content %}
+<h1>Bad request</h1>
+
+<p>You sent a request the server can’t understand. If you entered the URL manually please check your spelling and try again.</p>
+{% endblock %}