Event form fix 78188780

fixes the events new & edit forms so that they can manage the event's end date.
As part of this fix, also fixed some problems with the event model's end_time and schedule generation
I hope that the events model is solid now so that we can move forward more reliably.
https://www.pivotaltracker.com/s/projects/982890/stories/78188780
I would suggest using the `Hash#fetch` method: `params.fetch(:duration, @event.duration)`, makes it a little cleaner :grinning: