Aggressive LMTP header cleanup

This will remove all headers possible during LMTP delivery.

From: required or clients do not process the message correctly

Message-Id: required for clients to know which messages have been
downloaded

Chat-Is-Post-Message: is required for our attachment previews

Content-Type: required (??)

Subject: required due to test assertions

cmdeploy/src/cmdeploy/postfix/lmtp_header_cleanup

@@ -1,3 +1,6 @@
-/^DKIM-Signature:/            IGNORE
-/^Authentication-Results:/            IGNORE
-/^Received:/            IGNORE
+/^From:/            DUNNO
+/^Message-Id:/      DUNNO
+/^Chat-Is-Post-Message:/      DUNNO
+/^Content-Type:/    DUNNO
+/^Subject:/         DUNNO
+/.*/                IGNORE