Add textbook nav on the bottom of the page too

staticbook.html

@@ -57,6 +57,17 @@ function next_page() {
         </nav>
 
         <img id="bookpage" src="${ settings.BOOK_URL }p${ "%03i"%(page) }.png">
+
+        <nav class="bottom-nav">
+          <ul>
+            <li class="last">
+              <a href="javascript:prev_page()">Previous page</a>
+            </li>
+            <li class="next">
+              <a href="javascript:next_page()">Next page</a>
+            </li>
+          </ul>
+        </nav>
       </section>
     </section>
   </div>