Add paid_at date to donations

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

@@ -9,6 +9,7 @@
         <th>in EUR</th>
         <th>in USD</th>
         <th>Public name</th>
+        <th>Date</th>
         <th colspan="3"></th>
       </tr>
     </thead>
@@ -21,6 +22,7 @@
           <td><%= donation.amount_eur %></td>
           <td><%= donation.amount_usd %></td>
           <td><%= donation.public_name %></td>
+          <td><%= donation.paid_at ? donation.paid_at.strftime("%Y-%m-%d") : "" %></td>
           <td><%= link_to 'Show', admin_donation_path(donation) %></td>
           <td><%= link_to 'Edit', edit_admin_donation_path(donation) %></td>
           <td><%= link_to 'Destroy', admin_donation_path(donation), method: :delete, data: { confirm: 'Are you sure?' } %></td>