Feature/add sidekiq

Adds sidekiq background processing

• separate process running on heroku
• ActiveJobs and ActionMailers get pushed to Redis, sidekiqs reads and executes them
• admin view on staging: https://delvy-staging.herokuapp.com/sidekiq

unrelated fixes 💩 :

• fix email sending by enabling tls in smtp settings
• fix detached MR pipelines and allow merge without staging deployment