add mentions

app/templates/users/sign_up.html

@@ -4,7 +4,7 @@
 <div class="darkbg login-container">
   <h1>Sign up</h1>
   {% if inviter %}
-  <p>You have been invited by <a href="{{ url_for('users.page', username=inviter.username) }}">{{ inviter.username }}</a> to join {{ config.SITE_NAME }}. Create an identity below.</p>
+  <p>You have been invited by <a href="{{ url_for('users.page', username=inviter.username) }}">{{ inviter.get_readable_name() }}</a> to join {{ config.SITE_NAME }}. Create an identity below.</p>
   {% endif %}
   <form method="post">
     {% if key %}