Add ability for a venue to delete & edit concerts

@amZotti @maneyt @sherryseibel
Could you create a `.upcoming` method that combines `.next_first.after_today`? Like this:

```ruby
class Concert
  def self.upcoming
    next_first.after_today
  end
end
```

Or even add `.next_first` to your `after_today` method so it's always sorted next-first.