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 == HeraldRuleTypeConfig::RULE_TYPE_OBJECT;
}

public function getActionKey() {
return 'custom:add-risk';
}

public function getActionName() {
return 'Add risk rating (JSON)';
}

public function getActionType() {
return HeraldAdapter::VALUE_TEXT;
}

public function applyEffect(
HeraldAdapter $adapter,
$object,
HeraldEffect $effect) {

$key = "phragile:risk-rating";

// Read existing value.
$field_list = PhabricatorCustomField::getObjectFields(
$object,
PhabricatorCustomField::ROLE_VIEW);
$field_list->readFieldsFromStorage($object);
$field_list = mpull($field_list->getFields(), null, 'getFieldKey');
$field = $field_list[$key];
$field->setObject($object);
$field->setViewer(PhabricatorUser::getOmnipotentUser());

$risk = $field->getValue();
$old_risk = $risk; // PHP copies arrays by default!

// Add new value to array.
$herald_args = phutil_json_decode($effect->getTarget());
$risk[$herald_args['key']] = array(
'value' => $herald_args['value'],
'reason' => $herald_args['reason']);
$risk_key = $herald_args['key'];

// Set new value.
$adapter->queueTransaction(
id(new DifferentialTransaction())
->setTransactionType(PhabricatorTransactions::TYPE_CUSTOMFIELD)
->setMetadataValue('customfield:key', $key)
->setOldValue($old_risk)
->setNewValue($risk));

return new HeraldApplyTranscript(
$effect,
true,
pht(
'Modifying automatic risk ratings (key: %s)!',
$risk_key));
}

}
```

Test Plan: Created a custom action for differential revisions, set up a Herald rule to match and trigger the custom action, did 'arc diff' and saw the action trigger in the transcripts.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: locutus, edutibau, ite-klass, epriestley, Korvin

Maniphest Tasks: T4884

Differential Revision: https://secure.phabricator.com/D8784