Set up unborrow link for borrowings listed in shelf.htm

Basically, now when you click on the link it'll actually do the
stuff instead of just giving you some inaccurate notifications (no
offense, @abhiniype :) the notifications now *will* be accurate:
it's my delay, not yours).

growlinflask/templates/user/shelf.htm

@@ -35,7 +35,7 @@
     <span class="title"><b>{{r.copy.item.title}}</b></span>
     <p><i>{{r.borrow_date}}</i><br>
     </p>
-    <a href="#!" class="secondary-content tooltipped" data-position="left" data-delay="50" data-tooltip="Return">
+    <a href="{{ url_for('user_return', borrowid=r.id) }}" class="secondary-content tooltipped" data-position="left" data-delay="50" data-tooltip="Return">
       <i class="material-icons return" style="font-size:40px">done</i>
     </a>
   </div>