Event migration

Jon and Amal collaborated on this.
replaced the event model fields (event_date, start_time, end_time) 
with (start_datetime, duration).  
This removes the need for the from_must_come_before_to validation.
Many changes to accomodate tests, event _form view, events controller services (create and update), and more.
No migration test. I don't know if it's necessary, but if you insist, then I'll work on it.
I don't understand.  Can you please explain what the event_params filter is? And/or spell out how I should change this in more detail? 
In each case where this function is called, event_params is equivalent to: params[:event]