Generalize Legalpad validation logic for "Require Signature"

Summary:
See downstream <https://phabricator.wikimedia.org/T208254>.

I can't actually reproduce any issue here (we only show this field when creating a document, and only if the viewer is an administrator), so maybe this relied on some changes or was originally reported against older code.

Regardless, the validation isn't quite right: it requires administrator privileges to apply this transaction at all, but should only require administrator privileges to change the value.

Test Plan:
Edited Legalpad documents as an administrator and non-administrator before and after the change, with and without signatures being required.

Couldn't reproduce the original issue, but this version is generally more correct/robust.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20311

src/applications/legalpad/xaction/LegalpadDocumentRequireSignatureTransaction.php

@@ -55,11 +55,22 @@ final class LegalpadDocumentRequireSignatureTransaction
   public function validateTransactions($object, array $xactions) {
     $errors = array();
 
-    $is_admin = $this->getActor()->getIsAdmin();
+    $old = (bool)$object->getRequireSignature();
+    foreach ($xactions as $xaction) {
+      $new = (bool)$xaction->getNewValue();
 
+      if ($old === $new) {
+        continue;
+      }
+
+      $is_admin = $this->getActor()->getIsAdmin();
       if (!$is_admin) {
         $errors[] = $this->newInvalidError(
-        pht('Only admins may require signature.'));
+          pht(
+            'Only administrators may change whether a document '.
+            'requires a signature.'),
+          $xaction);
+      }
     }
 
     return $errors;