Calendar event update form submission should not lose date (except selected dates) when form has error.

Summary: Closes T8019, Calendar event update form submission should not lose date (except selected dates) when form has error.

Test Plan: Edit event, update date to invalid date, edit description, save, form should return an error and preserve the new description.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T8019

Differential Revision: https://secure.phabricator.com/D12643