[ticket/8558] Add display name in emails from board

PHPBB3-8558
[ticket/8558] Do string concatenation only once and do htmlspecialchars

PHPBB3-8558
Merge remote-tracking branch 'github-elsensee/ticket/stenudd/8558' into develop-ascraeus

* github-elsensee/ticket/stenudd/8558:
  [ticket/8558] Do string concatenation only once and do htmlspecialchars
  [ticket/8558] Add display name in emails from board