Avoid loading workers before setting Sidekiq defaults

What does this MR do?

Not a current bug, but it's possible that Sidekiq workers are referenced before having a chance to load its defaults.

It was seen at gitlab-com/gl-infra/scalability#356 (comment 340246119) that a simple API call in a initializer (rack_attack.rb) could cascade into loading a few Sidekiq workers without the default retry config.

525c6c04 indirectly solves this immediate issue for the Sidekiq retry config by removing this API reference, but other initializer could do the same later.

In order to avoid having this issue again, we move just the Sidekiq config that don't require the application classes to config/initializers_before_autoloader, so it'll make sure to load the configs without dependencies first.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content