Added Rubocop config for background migrations

What does this MR do?

This adds a Rubocop config file for background migrations. Reviewing background migrations can be a total pain since they're often quite complex. By adding these rules we can hopefully simplify this process a bit by forcing developers to simplify their code.

Does this MR meet the acceptance criteria?

• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

cc @smcgivern @DouweM