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

This PR replaces #1902 which was unfortunately not updated anymore by its author.

First I squashed his commits in only one commit and edited the commit message so it works. After that I removed the extra option in ACP because I thought that would be unnecessary. (Sitename should be enough) But since that reverted the changes he made in two files completely (and the changes would cause a conflict when merging in ascraeus) I squashed that change into his commit too.

After that I put that in one variable so `mail_encode` has to be called only once and added `htmlspecialchars`.

https://tracker.phpbb.com/browse/PHPBB3-8558