Addex X-GitLab-Project header to GitLab emails.

Fixes #8748.

app/mailers/notify.rb

@@ -111,6 +111,7 @@ class Notify < ActionMailer::Base
   # See: mail_answer_thread
   def mail_new_thread(model, headers = {}, &block)
     headers['Message-ID'] = message_id(model)
+    headers['X-GitLab-Project'] = "#{@project.name} | " if @project
     mail(headers, &block)
   end
@@ -125,6 +126,7 @@ class Notify < ActionMailer::Base
   def mail_answer_thread(model, headers = {}, &block)
     headers['In-Reply-To'] = message_id(model)
     headers['References'] = message_id(model)
+    headers['X-GitLab-Project'] = "#{@project.name} | " if @project
     if (headers[:subject])
       headers[:subject].prepend('Re: ')

spec/mailers/notify_spec.rb

@@ -26,6 +26,7 @@ describe Notify do
   shared_examples 'an email starting a new thread' do |message_id_prefix|
     it 'has a discussion identifier' do
       should have_header 'Message-ID',  /<#{message_id_prefix}(.*)@#{Gitlab.config.gitlab.host}>/
+      should have_header 'X-GitLab-Project', /#{project.name}/
     end
   end
@@ -37,6 +38,7 @@ describe Notify do
     it 'has headers that reference an existing thread' do
       should have_header 'References',  /<#{thread_id_prefix}(.*)@#{Gitlab.config.gitlab.host}>/
       should have_header 'In-Reply-To', /<#{thread_id_prefix}(.*)@#{Gitlab.config.gitlab.host}>/
+      should have_header 'X-GitLab-Project', /#{project.name}/
     end
   end