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, epriestley, gc3
Revert Plan:
Tags:

• begin *PUBLIC* platform impact section -

Bugzilla: #

• end platform impact -

Differential Revision: 357