Split application into functional parts to ensure that only needed code is loaded with all dependencies

Currently we run GitLab in multiple contexts:

• Web API
• Web ActionCable
• Web Controllers
• Sidekiq (and various different jobs)
• Sidekiq Cluster

Probably this kind of split is hard, but it appears that we could provide a better code architecture that would allow to split on this major line:

• Web: API/ActionCable/Controllers
• Sidekiq/Cluster

For example:

• Web: likely would not require Sidekiq Workers and all dependent services
• Sidekiq: likely would not require any API/ActionCable/Controllers/GraphQL and app dependencies to run