Add textbook nav on the bottom of the page too

sass/_textbook.scss

@@ -74,6 +74,13 @@ div.book-wrapper {
           }
         }
       }
+
+      &.bottom-nav {
+        margin-top: lh();
+        margin-bottom: -(lh());
+        border-bottom: 0;
+        border-top: 1px solid #EDDFAA;
+      }
     }
 
     section.page {

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>