Add rubocop in lefthook/CI

Context

I initially started with !1449 (closed), which became way to big to be reviewed properly.

I'm now breaking this down into manageable iterations 😄. This MR is the first iteration.

What does this MR do and why?

• Configures rubocop for triage-ops
• Adds a rubocop_todo.yml to be able to iterate on the offenses
• runs rubocop locally with lefthook
• runs rubocop in the CI - See the job that ran for this MR

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group => @gl-quality/eng-prod
  • Relevant Slack channels =>
  • Engineering week-in-review