Better wording for cancelling/reinstating recurring events

Summary: Fixes T10744

Test Plan: Create recurring event, cancel one instance, cancel the parent event, reinstate event. Wording in the reinstating dialog should be clear about reinstating only instances that haven't been individually cancelled.

Reviewers: chad, epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Maniphest Tasks: T10744

Differential Revision: https://secure.phabricator.com/D15770