Both ghost instances and exceptions to generated ghosts should be editable

Summary: Fixes T8369, Both ghost instances and exceptions to generated ghosts should be editable

Test Plan: Create recurring event, open /E{id}/1, edit, save. New exception should not be "recurring", and should be editable, as the regular ghost events.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8369

Differential Revision: https://secure.phabricator.com/D13089