I've set this up as a post-deploy background migration, taking advantage
of `each_batch` to set up jobs for Sidekiq workers to copy the project's
`merge_requests_require_code_owner_approval` to each of its protected
branch's `code_owner_approval_required` attribute. I've added some basic
tests here, as well.

I initially added a pair of #after_commit hooks to make sure projects
get updated while we were asynchronously chewing through the projects
and branches. However, I abandoned this, as a database reviewer supplied
a faster approach that shouldn't be an issue in terms of the total time
required to process the entities.