Allow Herald rules to apply "only the first time" to Calendar events

Summary: Fixes T12821. (Some events only happen once, so the default behavior doesn't let you pick this rule, and objects must opt into it by saying "yeah, I support multiple edits".)

Test Plan:
Note "only the first time" selected:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12821

Differential Revision: https://secure.phabricator.com/D18117