Add the “normal” link to campsite administration

This link is supposed to be for employees, to see the map and check
on campsites’ availability.  Currently, it shows the same for employees
and admins, but it will need to change.

web/templates/admin/layout.gohtml

@@ -48,6 +48,9 @@
             <li>
                 <a href="/admin/">{{( pgettext "Dashboard" "title" )}}</a>
             </li>
+            <li>
+                <a href="/admin/campsites">{{( pgettext "Campsites" "title" )}}</a>
+            </li>
         </ul>
     </nav>
 {{- end }}