tags: fix for problem with missing javascript

Signed-off-by: Eirini Psallida <eirini.psallida@cern.ch>
[.data()](http://api.jquery.com/data/) is nicer than using `.attr()` when dealing with [`data-*`](http://www.w3.org/html/wg/drafts/html/master/dom.html#embedding-custom-non-visible-data-with-the-data-*-attributes) attributes.