Revert "Delete mailer queue"

Procfile

 web: bundle exec unicorn_rails -p ${PORT:="3000"} -E ${RAILS_ENV:="development"} -c ${UNICORN_CONFIG:="config/unicorn.rb"}
-worker: bundle exec sidekiq -q post_receive,system_hook,project_web_hook,common,default,gitlab_shell
+worker: bundle exec sidekiq -q post_receive,mailer,system_hook,project_web_hook,common,default,gitlab_shell

bin/background_jobs

@@ -37,7 +37,7 @@ start_no_deamonize()
 
 start_sidekiq()
 {
-  bundle exec sidekiq -q post_receive -q system_hook -q project_web_hook -q gitlab_shell -q common -q default -e $RAILS_ENV -P $sidekiq_pidfile $@ >> $sidekiq_logfile 2>&1
+  bundle exec sidekiq -q post_receive -q mailer -q system_hook -q project_web_hook -q gitlab_shell -q common -q default -e $RAILS_ENV -P $sidekiq_pidfile $@ >> $sidekiq_logfile 2>&1
 }
 
 load_ok()