Surface custom form instructions as a "Remarkup" field for the transaction layer

Summary: Ref T13263. See <https://discourse.phabricator-community.org/t/image-uploads-for-forms-too-restricted-by-default/2594>. Currently, when you add instructions to a custom form, we don't expose them as remarkup, so `{Fxxx}` references don't get attached correctly and won't get proper permissions.

Test Plan:
Dragged-and-dropped an image into form instructions, saw the file attach to the form:

{F6367710}

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13263

Differential Revision: https://secure.phabricator.com/D20387

src/applications/transactions/storage/PhabricatorEditEngineConfigurationTransaction.php

@@ -149,5 +149,19 @@ final class PhabricatorEditEngineConfigurationTransaction
     return parent::getIcon();
   }
 
+  protected function newRemarkupChanges() {
+    $changes = array();
+
+    $type = $this->getTransactionType();
+    switch ($type) {
+      case self::TYPE_PREAMBLE:
+        $changes[] = $this->newRemarkupChange()
+          ->setOldValue($this->getOldValue())
+          ->setNewValue($this->getNewValue());
+        break;
+    }
+
+    return $changes;
+  }
 
 }