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.

spec/components/email/receiver_spec.rb

@@ -947,6 +947,52 @@ describe Email::Receiver do
         ordered_posts[1..-1].each(&:trash!)
         expect { process(:email_reply_4) }.to change { topic.posts.count }.by(1)
       end
+
+      describe "replying with various message-id formats" do
+        let!(:topic) do
+          process(:email_reply_1)
+          Topic.last
+        end
+        let!(:post) { Fabricate(:post, topic: topic) }
+
+        def process_mail_with_message_id(message_id)
+          mail_string = <<~REPLY
+          Return-Path: <two@foo.com>
+          From: Two <two@foo.com>
+          To: one@foo.com
+          Subject: RE: Testing email threading
+          Date: Fri, 15 Jan 2016 00:12:43 +0100
+          Message-ID: <44@foo.bar.mail>
+          In-Reply-To: <#{message_id}>
+          Mime-Version: 1.0
+          Content-Type: text/plain
+          Content-Transfer-Encoding: 7bit
+
+          This is email reply testing with Message-ID formats.
+          REPLY
+          Email::Receiver.new(mail_string).process!
+        end
+
+        it "posts a reply using a message-id in the format topic/TOPIC_ID/POST_ID@HOST" do
+          expect { process_mail_with_message_id("topic/#{topic.id}/#{post.id}@test.localhost") }.to change { Post.count }.by(1)
+          expect(topic.reload.posts.last.raw).to include("This is email reply testing with Message-ID formats")
+        end
+
+        it "posts a reply using a message-id in the format topic/TOPIC_ID@HOST" do
+          expect { process_mail_with_message_id("topic/#{topic.id}@test.localhost") }.to change { Post.count }.by(1)
+          expect(topic.reload.posts.last.raw).to include("This is email reply testing with Message-ID formats")
+        end
+
+        it "posts a reply using a message-id in the format topic/TOPIC_ID/POST_ID.RANDOM_SUFFIX@HOST" do
+          expect { process_mail_with_message_id("topic/#{topic.id}/#{post.id}.rjc3yr79834y@test.localhost") }.to change { Post.count }.by(1)
+          expect(topic.reload.posts.last.raw).to include("This is email reply testing with Message-ID formats")
+        end
+
+        it "posts a reply using a message-id in the format topic/TOPIC_ID.RANDOM_SUFFIX@HOST" do
+          expect { process_mail_with_message_id("topic/#{topic.id}/#{post.id}.x3487nxy877843x@test.localhost") }.to change { Post.count }.by(1)
+          expect(topic.reload.posts.last.raw).to include("This is email reply testing with Message-ID formats")
+        end
+      end
     end
 
     it "supports any kind of attachments when 'allow_all_attachments_for_group_messages' is enabled" do
@@ -1161,6 +1207,7 @@ describe Email::Receiver do
         NotificationEmailer.enable
         SiteSetting.disallow_reply_by_email_after_days = 10000
         Jobs.run_immediately!
+        Email::MessageIdService.stubs(:random_suffix).returns("blah123")
       end
 
       def reply_as_group_user
@@ -1185,7 +1232,7 @@ describe Email::Receiver do
 
       it "creates an EmailLog when someone from the group replies, and does not create an IncomingEmail record for the reply" do
         email_log, group_post = reply_as_group_user
-        expect(email_log.message_id).to eq("topic/#{original_inbound_email_topic.id}/#{group_post.id}@test.localhost")
+        expect(email_log.message_id).to eq("topic/#{original_inbound_email_topic.id}/#{group_post.id}.blah123@test.localhost")
         expect(email_log.to_address).to eq("two@foo.com")
         expect(email_log.email_type).to eq("user_private_message")
         expect(email_log.post_id).to eq(group_post.id)