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:

{F4999093}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12821

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

src/applications/calendar/herald/PhabricatorCalendarEventHeraldAdapter.php

@@ -49,6 +49,13 @@ final class PhabricatorCalendarEventHeraldAdapter extends HeraldAdapter {
     }
   }
 
+  public function getRepetitionOptions() {
+    return array(
+      HeraldRepetitionPolicyConfig::EVERY,
+      HeraldRepetitionPolicyConfig::FIRST,
+    );
+  }
+
   public function getHeraldName() {
     return $this->getObject()->getMonogram();
   }