Fix Message-ID field to be RFC 2111-compliant to prevent e-mails from being dropped

Closes #2867

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.1.0 (unreleased)
+  - Fix Message-ID header to be RFC 2111-compliant to prevent e-mails being dropped (Stan Hu)
   - Add user preference to view activities as default dashboard (Stan Hu)
   - Add option to admin area to sign in as a specific user (Pavel Forkert)
   - Show CI status on all pages where commits list is rendered

app/mailers/notify.rb

@@ -140,7 +140,7 @@ class Notify < BaseMailer
   #  * have a 'In-Reply-To' or 'References' header that references the original 'Message-ID'
   #
   def mail_answer_thread(model, headers = {})
-    headers['Message-ID'] = SecureRandom.hex
+    headers['Message-ID'] = "<#{SecureRandom.hex}@#{Gitlab.config.gitlab.host}>"
     headers['In-Reply-To'] = message_id(model)
     headers['References'] = message_id(model)
 

spec/mailers/notify_spec.rb

@@ -52,6 +52,7 @@ describe Notify do
     end
 
     it 'has headers that reference an existing thread' do
+      is_expected.to have_header 'Message-ID',  /<(.*)@#{Gitlab.config.gitlab.host}>/
       is_expected.to have_header 'References',  /<#{thread_id_prefix}(.*)@#{Gitlab.config.gitlab.host}>/
       is_expected.to have_header 'In-Reply-To', /<#{thread_id_prefix}(.*)@#{Gitlab.config.gitlab.host}>/
       is_expected.to have_header 'X-GitLab-Project', /#{project.name}/