message: Add alt tags to the logo images on the 404 page
diff: <div id="nav-header-bg"></div>
  <div id="header-main" class="container">
  <a href="/" class="translatable">
  - <img id="logo" class="translatable-en" src="/img/logo-datarequests.svg">
  - <img id="logo" class="translatable-de" src="/img/logo-datenanfragen.svg">
  - <img id="logo" class="translatable-fr" src="/img/logo-demandetesdonnees.svg">
  - <img id="logo" class="translatable-pt" src="/img/logo-pedidodedados.svg">
  + <img id="logo" class="translatable-en" src="/img/logo-datarequests.svg" alt="datarequests.org">
  + <img id="logo" class="translatable-de" src="/img/logo-datenanfragen.svg" alt="Datenanfragen.de">
  + <img id="logo" class="translatable-fr" src="/img/logo-demandetesdonnees.svg" alt="demandetesdonnees.fr">
  + <img id="logo" class="translatable-pt" src="/img/logo-pedidodedados.svg" alt="pedidodedados.org">
  </a>
  -
  <div class="clearfix"></div>
  </div>
  </header>
  