Replace invalid use of 'rel' attribute with data attributes.

The blocklayered module uses `rel` attributes to inject data into the DOm elements that are used in Javascript to manipulate filters. This is a violation of the HTML5 spec, which [explicitly declares](http://www.w3.org/TR/html5/links.html#linkTypes) the allowed values for `rel` attributes.

The correct way to inject data into the DOM is using `data-` attributes. This patch does that.