Recognize Maniphest task description as a remarkup block

Summary:
Ref T4589. We don't recognize task descriptions as remarkup blocks, so `{F...}` references in them do not get attached to the objects, and thus no policy exemption is created.

Recognize them, which activates `{F...}` and `@mentions`.

We probably have a few more of these in other applications, but it's not a big deal to clean them up as they arise.

Test Plan: Uploaded a file to a task in the description field, saw it attach and get a policy exemption.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4589

Differential Revision: https://secure.phabricator.com/D10139

src/applications/maniphest/storage/ManiphestTransaction.php

@@ -43,6 +43,18 @@ final class ManiphestTransaction
     return parent::shouldGenerateOldValue();
   }
 
+  public function getRemarkupBlocks() {
+    $blocks = parent::getRemarkupBlocks();
+
+    switch ($this->getTransactionType()) {
+      case self::TYPE_DESCRIPTION:
+        $blocks[] = $this->getNewValue();
+        break;
+    }
+
+    return $blocks;
+  }
+
   public function getRequiredHandlePHIDs() {
     $phids = parent::getRequiredHandlePHIDs();