confirmation: Create render_confirmation_key_error function.

templates/confirmation/link_malformed.html

@@ -0,0 +1,20 @@
+{% extends "zerver/portico.html" %}
+
+{% block portico_content %}
+
+<div class="pitch">
+    <hr/>
+    <p class="lead">Whoops. The confirmation link is malformed.</p>
+
+    <p>Make sure you copied the link correctly in to your browser. If you're
+    still encountering this page, it's probably our fault. We're sorry.</p>
+
+    <p>
+        Anyway, shoot us a line at
+        <a href="mailto:{{ support_email }}">{{ support_email }}</a>
+        and we'll get this resolved shortly.
+    </p>
+
+</div>
+
+{% endblock %}