Move IrkerWorker into Integrations:: namespace and rename queue

What does this MR do and why?

For backwards-compatibility we're keeping the old worker and queue around for another release.

Also clean up all Rubocop offenses in the worker, model, and specs.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #330670 (closed)