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/aphront/AphrontRequest.php

@@ -353,7 +353,16 @@ final class AphrontRequest {
    * @return  dict<string, string>  Original request parameters.
    */
   public function getPassthroughRequestParameters() {
-    $data = self::flattenData($this->getRequestData());
+    return self::flattenData($this->getPassthruRequestData());
+  }
+
+  /**
+   * Get request data other than "magic" parameters.
+   *
+   * @return dict<string, wild> Request data, with magic filtered out.
+   */
+  public function getPassthroughRequestData() {
+    $data = $this->getRequestData();
 
     // Remove magic parameters like __dialog__ and __ajax__.
     foreach ($data as $key => $value) {