R1:22c1b386558e

herald: add the ability to execute a rule the first time only

Summary:
- added a new config class for representing the kind of repetition a rule has
(once, every time, first time only)

- added an email action to herald rules for differential to allow someone to get
an email but only the first one

- changed the herald rule ui to allow a user to pick the amount of repetition

Test Plan:
created a test rule and ran it over and over

Reviewed By: epriestley
Reviewers: epriestley, tuomaspelkonen
CC: aran…