Issue warning if the project CTF was generated pre-Unicode

PP artifacts that were generated before the Unicode conversion still
have their contents in Latin-1. There aren't many of these but we
periodically stumble over some that are reclaimed.

Author: cpeel

project.php

@@ -1652,6 +1652,11 @@ function echo_download_zip(string $link_text, string $discriminator): void
     echo "</a>";
     echo_byte_size($filesize_b);
     echo_last_modified($last_modified);
+    // output a warning if the file was generated before the UTF-8 cutover.
+    // timestamp is midnight May 18 2020
+    if ($discriminator == "" && $last_modified && $last_modified < 1589785200) {
+        echo "<br><span class='warning'>" . _("This file was generated before the Unicode conversion. If you are just starting to PP this project, please ask a site administrator to regenerate the file.") . "</span>";
+    }
     echo "</li>";
     echo "\n";
 }