Add cancelled-invite html.

If a invited user accepts a cancelled invitation they are directed to a page telling them the invitation is cancelled.
Without this they were able to register and were added to the service.

app/templates/views/cancelled-invitation.html

@@ -0,0 +1,14 @@
+{% extends "withoutnav_template.html" %}
+{% block page_title %}Invitation has been cancelled{% endblock %}
+{% block maincolumn_content %}
+    <div class="grid-row">
+      <div class="column-two-thirds">
+        <h1 class="heading-large">
+          The invitation you were sent has been cancelled.
+        </h1>
+        <p>
+          The person that sent you the invitation decided that it was sent in error and has cancelled the invitation.
+        </p>
+      </div>
+    </div>
+{% endblock %}