Improves query efficiency for shelve buttons

About 50 fewer queries (yikes)

bookwyrm/templates/get_started/book_preview.html

@@ -5,7 +5,7 @@
     <div class="select is-small mt-1 mb-3">
         <select name="{{ book.id }}" aria-label="{% blocktrans with book_title=book.title %}Have you read {{ book_title }}?{% endblocktrans %}">
             <option disabled selected value>Add to your books</option>
-            {% for shelf in request.user.shelf_set.all %}
+            {% for shelf in user_shelves %}
             <option value="{{ shelf.id }}">{{ shelf.name }}</option>
             {% endfor %}
         </select>