Provide an Editor extension point for transaction validation

Summary:
Depends on D20040. Ref T13242. See PHI1039. See PHI873. Two reasonable cases have arisen recently where extending validation rules would help solve problems.

We can do this in a pretty straightforward way with a standard extension pattern.

Test Plan:
Used this extension to keep ducks away from projects:

```lang=php
<?php

final class NoDucksEditorExtension
extends PhabricatorEditorExtension {

const EXTENSIONKEY = 'no.ducks';

public function getExtensionName() {
return pht('No Ducks!');
}

public function supportsObject(
PhabricatorApplicationTransactionEditor $editor,
PhabricatorApplicationTransactionInterface $object) {
return ($object instanceof PhabricatorProject);
}

public function validateTransactions($object, array $xactions) {
$errors = array();

$name_type = PhabricatorProjectNameTransaction::TRANSACTIONTYPE;

$old_value = $object->getName();
foreach ($xactions as $xaction) {
if ($xaction->getTransactionType() !== $name_type) {
continue;
}

$new_value = $xaction->getNewValue();
if ($old_value === $new_value) {
continue;
}

if (preg_match('/duck/i', $new_value)) {
$errors[] = $this->newInvalidTransactionError(
$xaction,
pht('Project names must not contain the substring "duck".'));
}
}

return $errors;
}

}
```

{F6162585}

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13242

Differential Revision: https://secure.phabricator.com/D20041