email-mirror: Add a standalone server that processes incoming email.

Using postfix to handle the incoming email gateway complicates things
a great deal:

- It cannot verify that incoming email addresses exist in Zulip before
  accepting them; it thus accepts mail at the `RCPT TO` stage which it
  cannot handle, and thus must reject after the `DATA`.

- It is built to handle both incoming and outgoing email, which
  results in subtle errors (1c17583ad5, 79931051bd, a53092687e,
  #18600).

- Rate-limiting happens much too late to avoid denial of
  service (#12501).

- Mis-configurations of the HTTP endpoint can break incoming
  mail (#18105).

Provide a replacement SMTP server which accepts incoming email on port
25, verifies that Zulip can accept the address, and that no
rate-limits are being broken, and then adds it directly to the
relevant queue.

Removes an incorrect comment which implied that missed-message
addresses were only usable once.  We leave rate-limiting to only
channel email addresses, since missed-message addresses are unlikely
to be placed into automated systems, as channel email addresses are.

Also simplifies #7814 somewhat.

tools/linter_lib/custom_check.py

@@ -38,6 +38,8 @@ FILES_WITH_LEGACY_SUBJECT = {
     # to fix everything until we migrate the DB to "topic".
     "zerver/tests/test_message_fetch.py",
     "zerver/tests/test_message_topics.py",
+    # This is actually email subjects
+    "zerver/lib/email_mirror_server.py",
 }
 
 shebang_rules: list["Rule"] = [