Check if recurrence end date is disabled before saving it.
Summary: Ref T8357, Check if recurrence end date is disabled before saving it.
Test Plan: Create new event, before saving, leave "recurrence end date" unchecked, save, should not get an error.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: epriestley, Korvin
Maniphest Tasks: T8357
Differential Revision: https://secure.phabricator.com/D13212