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 don't know how to 'sanitize' or 'validate' the attributes before they get here.  Especially the datetime.  In any case, I don't have the time to look into it.  
The tests on lines 138-150 do test the case of invalid attributes (empty title, no start_datetime) for update.  So I think that this is good enough.