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
This is sad. We could do some elaborate polymorphism, I guess, but that's just beheading the hydra.