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.
Yes.  I think that Amal uses Vagrant,and when he committed, this file and others must have been part of the commit. How do I remove these files from the PR?  I don't think that I even have these files on my machine.