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

resources/celerity/map.php

@@ -146,7 +146,7 @@ return array(
     'rsrc/css/phui/phui-comment-panel.css' => 'f50152ad',
     'rsrc/css/phui/phui-crumbs-view.css' => '10728aaa',
     'rsrc/css/phui/phui-curtain-view.css' => '2bdaf026',
-    'rsrc/css/phui/phui-document-pro.css' => '1a08ef4b',
+    'rsrc/css/phui/phui-document-pro.css' => 'd033e8d5',
     'rsrc/css/phui/phui-document-summary.css' => '9ca48bdf',
     'rsrc/css/phui/phui-document.css' => 'c4ac41f9',
     'rsrc/css/phui/phui-feed-story.css' => '44a9c8e9',
@@ -814,7 +814,7 @@ return array(
     'phui-curtain-view-css' => '2bdaf026',
     'phui-document-summary-view-css' => '9ca48bdf',
     'phui-document-view-css' => 'c4ac41f9',
-    'phui-document-view-pro-css' => '1a08ef4b',
+    'phui-document-view-pro-css' => 'd033e8d5',
     'phui-feed-story-css' => '44a9c8e9',
     'phui-font-icon-base-css' => '870a7360',
     'phui-fontkit-css' => '1320ed01',