Events#index displays currently-running events

* Add class methods to `Event` to display current and closed events
* Show lists of current and closed events on `events#index`
* Update `_event` partial to show relative closing date
* Extract helper method for showing closing date relative to today

https://trello.com/c/amqWgDDf
Yeah, I'm not thrilled about it, but it seems like the better alternative. I think it's *reasonably* readable.