R1:88aba65d54c2

Support custom actions in Herald

Summary:
This was significantly easier than expected. Here's an example of what an extension class might look like:

```
<?php

final class AddRiskReviewHeraldCustomAction extends HeraldCustomAction {

public function appliesToAdapter(HeraldAdapter $adapter) {
return $adapter instanceof HeraldDifferentialRevisionAdapter;
}

public function appliesToRuleType($rule_type) {
return $rule_type == HeraldRuleTypeConfig::RULE_TYPE_GLOBAL ||
$rule_type ==…