template: lie le label au champ de recherche pour améliorer l'accessibilité

Fixes #11

app/templates/search/search-form.html

 <label class="visually-hidden" for="keywords">Mots-clés, "expression exacte"</label>
 <div class="input-group">
-  <input type="text" class="form-control form-control-lg" name="s" placeholder='Mots-clés, "expression exacte"' value="{{ search }}">
+  <input id="keywords" type="text" class="form-control form-control-lg" name="s" placeholder='Mots-clés, "expression exacte"' value="{{ search }}">
   <button type="submit" class="btn btn-lg btn-primary d-none d-md-inline">Rechercher</button>
 </div>
 <div class="input-group w-100 py-2">