Improve some commenting/editing behaviors for recurring events

Summary:
Ref T11809. Currently, commenting on a recurring event hits the same "one or all?" dialog that other edits do.

For comments and edits submitted via the comment widget, we can safely assume that you mean "just this one", since it doesn't really make sense to try to bulk-edit an event from that UI.

Test Plan: Commented on a recurring event parent and an event in the series.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11809

Differential Revision: https://secure.phabricator.com/D16795