Include Link header and element in invite response

This allows future App versions to also work with the invite page
without having to screen scrape the content.

Fixes #56 (at least for the portal side of things).

snikket_web/templates/invite_view.html

@@ -6,6 +6,7 @@
 <title>{% trans site_name=config["SITE_NAME"] %}Invite to {{ site_name }} | Snikket{% endtrans %}</title>
 <script async type="text/javascript" src="{{ url_for("static", filename="js/invite-magic.js") }}"></script>
 <script async type="text/javascript" src="{{ url_for("static", filename="js/qrcode.min.js") }}"></script>
+<link rel="alternate" href="{{ invite.xmpp_uri }}">
 {% endblock %}
 {% block content %}
 <div class="elevated box el-3">