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

resources/sql/patches/041.heraldrepetition.sql

@@ -0,0 +1,7 @@
+CREATE TABLE phabricator_herald.herald_ruleapplied (
+  ruleID int unsigned not null,
+  phid varchar(64) binary not null,
+  PRIMARY KEY(ruleID, phid)
+) ENGINE=InnoDB;
+
+ALTER TABLE phabricator_herald.herald_rule add repetitionPolicy int unsigned;