Improve table styles

* Nicer table headers
* Hide invitation IDs on small screens

app/views/invitations/index.html.erb

@@ -29,7 +29,7 @@
     <table>
       <thead>
         <tr>
-          <th>URL</th>
+          <th class="hide-small">ID</th>
           <th>Accepted</th>
           <th>Invited user</th>
         </tr>
@@ -37,7 +37,7 @@
       <tbody>
         <% @invitations_used.each do |invitation| %>
           <tr>
-            <td class="overflow-ellipsis"><%= invitation_url(invitation.token) %></td>
+            <td class="hide-small"><%= invitation.token %></td>
             <td><%= invitation.used_at.strftime("%Y-%m-%d") %></td>
             <td><%= User.find(invitation.invited_user_id).address %></td>
           </tr>