Move backend jQuery plugins to dedicated subdirectory.

From now on, we'll keep jQuery plugins required by the backend in
/templates/2k11/admin/js/ (as well as other vendor JS like e.g.
Modernizr).

templates/2k11/admin/media_choose.tpl

@@ -47,7 +47,7 @@
             display: block;
         } 
     </style>
-    <script src="{serendipity_getFile file='dragdrop.js'}" ></script>
+    <script src="{serendipity_getFile file="admin/js/dragdrop.js"}"></script>
     <script src="{serendipity_getFile file='admin/imgedit.js'}" ></script>
 {/if}
     <script src="{serendipity_getFile file='YahooUI/treeview/YAHOO.js'}"></script>