DRAFT, recurring events need optional end dates

Summary: Ref T8357, DRAFT, recurring events need optional end dates

Test Plan: Edit recurring event, set end date, save, recurring ghosts should not generate after end date

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8357

Differential Revision: https://secure.phabricator.com/D13088