fire resize when gallery lightbox closes

Resizing in lightbox mode breaks imager's responsive image loading (as the element containing the images is `display: none;`). Fire a resize when lightbox closes.

it's listening to mediator's event - https://github.com/guardian/frontend/blob/master/common/app/assets/javascripts/modules/ui/images.js#L50 - so might as well fire that directly

`mediator.emit('window:resize');`