in DSNs, add a References header pointing to the message with deliverability issues

so mail user agents will show DSNs threaded/grouped with the original message.
we store the MessageID in the message queue, so we have the value within reach
when we need it.

i saw a references header in a DSN from gmail on a test account. makes sense to me.

http/adminapi.json

@@ -2945,6 +2945,13 @@
 						"int64"
 					]
 				},
+				{
+					"Name": "MessageID",
+					"Docs": "Used when composing a DSN, in its References header.",
+					"Typewords": [
+						"string"
+					]
+				},
 				{
 					"Name": "MsgPrefix",
 					"Docs": "",