Fix #2287 (books can't be added to shelf from search)

cps/templates/search.html

@@ -9,6 +9,7 @@ <h2>{{result_count}} {{_('Results for:')}} {{adv_searchterm}}</h2>
       {% if g.user.is_authenticated %}
         {% if g.user.shelf.all() or g.shelves_access %}
           <div id="shelf-actions" class="btn-toolbar" role="toolbar">
+            <input type="hidden" name="csrf_token" value="{{ csrf_token() }}">
             <div class="btn-group" role="group" aria-label="Add to shelves">
               <button id="add-to-shelf" type="button" class="btn btn-primary btn-sm dropdown-toggle" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
                 <span class="glyphicon glyphicon-list"></span> {{_('Add to shelf')}}