Support (basic) commenting on Phriction documents

Summary:
Depends on D19659. Fixes T1894. Ref T13077. See PHI840.

  - Add an EditEngine, although it currently supports no fields.
  - Add (basic, top-level-only) commenting (we already had the table in the database).

This will probably create some issues. I'm most concerned about documents accumulating a ton of old, irrelevant comments over time which are hard to keep track of and no longer relevant. But I think this is probably a step forward in almost all cases, and a good thing on the balance.

This also moves us incrementally toward putting all editing on top of EditEngine.

Test Plan: {F5877347}

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13077, T1894

Differential Revision: https://secure.phabricator.com/D19660

src/applications/phriction/controller/PhrictionEditEngineController.php

@@ -0,0 +1,14 @@
+<?php
+
+final class PhrictionEditEngineController
+  extends PhrictionController {
+
+  public function handleRequest(AphrontRequest $request) {
+    // NOTE: For now, this controller is only used to handle comments.
+
+    return id(new PhrictionDocumentEditEngine())
+      ->setController($this)
+      ->buildResponse();
+  }
+
+}