Split off from, but related to, https://tracker.phpbb.com/browse/CUSTDB-826

Received an email with 🖌️ 📐 in the subject line, where the actual topic title has 🖌️ 📐.

Email subject lines have their own means of encoding non-ASCII text, which is unrelated to HTML entities, so HTML entities should never be appearing in that context.

The email template was `topic_notify.txt` (template variables injected by `phpbb/notification/type/post.php`).

PR up shortly.

(Please note if quoting in a reply, posting actual emojis will break Jira, so you need to double-encode them, i.e. it will look like 🖌️. And what I just typed there was 🖌️. And that was 🖌️... and so on ad infinitum).