implement downloads

cops-web/templates/book_card.html

@@ -0,0 +1,25 @@
+ <article class="book-card">
+  <header class="grid-item">
+    <hgroup>
+      <h5>{{ book.title }}</h5>
+      <p>{{ book.author }}</p>
+    </hgroup>
+  </header>
+  <img class="cover" src="/cover/{{ book.id }}" alt="book cover">
+  <footer>
+    <form>
+      <fieldset role="group">
+        <details class="dropdown">
+          <summary role="button" class="outline">
+            Download 
+          </summary>
+          <ul>
+          {% for format, _ in book.formats %}
+            <li><a href="/book/{{ book.id }}/{{ format }}">{{ format }}</a></li>
+          {% endfor %}
+          </ul>
+        </details>
+      </fieldset>
+    </form>
+  </footer>
+</article>