Add draft support to ApplicationTransactions

Summary:
When previewing, save drafts. When loading objects, restore drafts if they are available.

Depends on: D665

Test Plan:
  - Viewed a Mock.
  - Typed text into the comment box.
  - Reloaded the page.
  - Text still there.
  - Hit submit, got my comment.
  - Reloaded the page.
  - Draft correctly deleted.
  - Repeated for Macros.

Reviewers: btrahan, chad, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2104

Differential Revision: https://secure.phabricator.com/D4252

src/applications/macro/controller/PhabricatorMacroCommentController.php

@@ -23,6 +23,7 @@ final class PhabricatorMacroCommentController
     }
 
     $is_preview = $request->isPreviewRequest();
+    $draft = PhabricatorDraft::buildFromRequest($request);
 
     $view_uri = $this->getApplicationURI('/view/'.$macro->getID().'/');
 
@@ -52,6 +53,10 @@ final class PhabricatorMacroCommentController
         ->setException($ex);
     }
 
+    if ($draft) {
+      $draft->replaceOrDelete();
+    }
+
     if ($request->isAjax()) {
       return id(new PhabricatorApplicationTransactionResponse())
         ->setViewer($user)