Replace subject with [...] for outgoing mail

`authclean` cleanup server is used by
reinjecting smtpd running on localhost:10025 by default.
It runs after filtermail
and currently removes `Received` header
to avoid leaking IP address.
Can as well be used to replace `Subject` lines
with `Subject: [...]`.
If there are multiple `Subject` lines,
all of them should be replaced.

This allows us to avoid dealing with
localized subjects, including SecureJoin
messages `vc-request` and `vg-request`
which can have Subject lines like
Subject: =?utf-8?q?Nachricht_von_nrn178fi4=40nine=2Etestrun=2Eorg?=

CHANGELOG.md

@@ -17,6 +17,9 @@
 - remove MTA-STS daemon
   ([#488](https://github.com/deltachat/chatmail/pull/488))
 
+- replace `Subject` with `[...]` for all outgoing mails.
+  ([#481](https://github.com/deltachat/chatmail/pull/481))
+
 ## 1.5.0 2024-12-20
 
 - cmdeploy dns: always show recommended DNS records