R1:f7e8fa0764fe

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'…