Add invitation stats

Show some stats about unused invitations and active inviters

app/views/admin/invitations/index.html.erb

@@ -1,7 +1,14 @@
 <section>
   <h2>Invitations</h2>
-
-  <% if @invitations_used.any? %>
+  <p>
+    There are currently <strong><%= @invitations_unused_count %>
+    unused invitations</strong> available to existing users.
+    <strong><%= @users_with_referrals_count %> users</strong> have successfully
+    invited new users.
+  </p>
+</section>
+<% if @invitations_used.any? %>
+  <section>
     <h3>Accepted (<%= @invitations_used.length %>)</h3>
     <table>
       <thead>
@@ -21,5 +28,5 @@
         <% end %>
       </tbody>
     </table>
-  <% end %>
-</section>
+  </section>
+<% end %>