Allow safe editing of recurring events

Summary: Ref T8472, Allow safe editing of recurring events

Test Plan: Create recurring event, edit event, everything but date-related fields should be editable

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8472

Differential Revision: https://secure.phabricator.com/D13230