admin: Add registration time to dashboard table

Does this because "last_login" isn't actually set when people log in. It
should be fixed for next year.

goathacks/templates/admin.html

@@ -254,7 +254,7 @@
                         <td id="{{ hacker['id'] }}-waitlisted">{{ hacker['waitlisted'] }}</td>
                         <td>{{ hacker.is_admin }}</td>
                         <td>{{ hacker['id'] }}</td>
-                        <td>{{ hacker['registration_time'] }}</td>
+                        <td>{{ hacker['last_login'] }}</td>
                         <td>{{ hacker['email'] }}</td>
                         <td>{{ hacker['first_name'] + ' ' + hacker['last_name'] }}</td>
                         <td>{{ hacker['phone'] }}</td>