Ensure onCollapseButtonClick is only called on target Card

Currently if you open up the Sessions card, click on an Event, then hit the escape key, you'll be stuck on the Sessions view. This is because `onCollapseButtonClick` will fire on both the Event card and the Sessions card, meaning that the browser will navigate up two directories to the homepage (i.e. `/devsummit/sessions/making-web-apps-appy/` -> `/devsummit/`) but leave the Sessions card open. At this point, the user will not be able to navigate back to the homepage since `this.expanded_` has been set to false on the Event card even though it's still visible.

This PR fixes this issue by assigning an `id` to each card equal to the card's `href` attribute (this is already being done internally in the `cards` object inside of `components/cards.js`). When `onKeyUp_` is triggered, a check is made to see if the current url is equal to the card we're trying to collapse's `id`. If it is not, the event is ignored, preventing multiple `onCollapseButtonClick` from firing on multiple cards.