Add a header warning to revisions that need a legal document signature

Summary: This supplements the footer warning and makes it more visible for authors.

Test Plan: {F173277}

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D9794

src/applications/differential/customfield/DifferentialRequiredSignaturesField.php

···
     return false;
   }
 
+  public function getWarningsForRevisionHeader(array $handles) {
+    if (!$this->haveAnyUnsignedDocuments()) {
+      return array();
+    }
+
+    return array(
+      pht(
+        'This revision can not be accepted until the required legal '.
+        'agreements have been signed.'),
+    );
+  }
+
 
 }