rename .html templates to .jinja2 for more consistency

templates/backlinks.html

@@ -1,27 +0,0 @@
-{% extends "base.html" %}
-
-{% block title %}Pages linking to “{{ p.title }}” - {{ app_name }}{% endblock %}
-
-{% block meta %}
-<meta name="robots" content="noindex,nofollow" />
-{% endblock %}
-
-{% block content %}
-<h1 id="firstHeading">{{ p.title }}</h1>
-<div class="preview-subtitle">{{ T('backlinks') }}</div>
-
-{% if backlinks %}
-<ul>
-{% for op in backlinks %}
-  <li><a href="{{ op.get_url() }}">
-    {{ op.title }} (<strong>#{{ op.id }}</strong>)
-  </a></li>
-{% endfor %}
-</ul>
-{% else %}
-<p class="placeholder">{{ T("backlinks-empty") }}</p>
-{% endif %}
-
-<p>{{ T("back-to") }} <a href="{{ p.get_url() }}">{{ p.title }}</a>.</p>
-{% endblock %}
-