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?=

chatmaild/src/chatmaild/tests/plugin.py

@@ -69,7 +69,7 @@ def maildata(request):
 
     assert datadir.exists(), datadir
 
-    def maildata(name, from_addr, to_addr, subject="..."):
+    def maildata(name, from_addr, to_addr, subject="[...]"):
         # Using `.read_bytes().decode()` instead of `.read_text()` to preserve newlines.
         data = datadir.joinpath(name).read_bytes().decode()