Bulk actions support for events

`itemStash.expire` will show the seconds left to the expiration, which might not be too useful, particularly when > 3600.

We could have a method that returns the expiration in a human-readable way (such as `28 seconds`, `46 minutes` or `14 hours`) or use the existing [getExpireTimestamp](https://github.com/sensu/uchiwa/blob/1bd03ea3389a8d33077257380fdcce9ce80103f8/public/js/filters.js#L89) filter, currently used in the stashes page.

Makes sense?