FIX: Add random suffix to outbound Message-ID for email (#15179)

Currently the Message-IDs we send out for outbound email
are not unique; for a post they look like:

topic/TOPIC_ID/POST_ID@HOST

And for a topic they look like:

topic/TOPIC_ID@HOST

This commit changes the outbound Message-IDs to also have
a random suffix before the host, so the new format is
like this:

topic/TOPIC_ID/POST_ID.RANDOM_SUFFIX@HOST

Or:

topic/TOPIC_ID.RANDOM_SUFFIX@HOST

This should help with email deliverability. This change
is backwards-compatible, the old Message-ID format will
still be recognized in the mail receiver flow, so people
will still be able to reply using Message-IDs, In-Reply-To,
and References headers that have already been sent.

This commit also refactors Message-ID related logic
to a central location, and adds judicious amounts of
tests and documentation.

app/controllers/webhooks_controller.rb

@@ -13,7 +13,7 @@ class WebhooksController < ActionController::Base
   def sendgrid
     events = params["_json"] || [params]
     events.each do |event|
-      message_id = Email.message_id_clean((event["smtp-id"] || ""))
+      message_id = Email::MessageIdService.message_id_clean((event["smtp-id"] || ""))
       to_address = event["email"]
       if event["event"] == "bounce"
         if event["status"]["4."]
@@ -150,7 +150,7 @@ class WebhooksController < ActionController::Base
     return mailgun_failure unless valid_mailgun_signature?(params["token"], params["timestamp"], params["signature"])
 
     event = params["event"]
-    message_id = Email.message_id_clean(params["Message-Id"])
+    message_id = Email::MessageIdService.message_id_clean(params["Message-Id"])
     to_address = params["recipient"]
 
     # only handle soft bounces, because hard bounces are also handled