Use PlainTextOutput::renderFromHtml for email subject

I just stumbled on this issue, in our project we use symfony_mailer_lite and print {{ subject }} in the twig mail template.
Twig encodes these chars to HTML entities: '&<>
If the subject contains any of these chars, then they get printed as encoded in the final twig template/email.

This is not an issue for the actual mail subject as it goes through PlainTextOutput::renderFromHtml() here.
But the twig template is rendered just before that, from here.
So I believe it makes sense to apply the proposed suggestion.

herved → closed merge request !22

Actually doing this would cause subject to go twice through strip_tags and html_entity_decode...