Some email clients (notably, Gmail Web) support automatically threading
emails together if recipients and subjects match[1]. Manual testing
indicated that prefixing a subject with "[bracketed content]" does not
break this threading behavior, but the added checkmark in a resolved
topic's title does. Before sending an email notification, determine
whether the topic is resolved, and pass this information to the Jinja
template to properly format a threadable email subject.
Fixes: #22538
[1]: https://support.google.com/mail/answer/5900
