Implement edit_event endpoint

Updated the route and view to behavior similar to the add event
endpoint.
Rename add_form.js to event_form.js

This entry script is used on both the add and edit event page.
Add UI test for edit event form

This tests to make sure that datetime fields are converted from UTC to
localtime when populating the form. This is important so that when you
save the form without any modifications, your datetime values don't
increase by the local timezone offset.