Event datetime migration

Event migration- 
Replaced the event model fields (event_date, start_time, end_time) with (start_datetime, duration)- Pivotal#: 75570752
Modified the Event _form view, used by New and Edit, to use these two new fields.  start_datetime is presently split into separate datepicker and timepicker.
We also wrote a new rspec test for the migration.
Collaborated with Amal and Yaro.