Add Rubocop for Rails.logger

As mentioned in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23031/diffs, we should avoid using Rails.logger and move ahead with structured logs.

We should add a Rubocop that bans this.

/cc: @grzesiek

Merge Requests

• CE: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30415
• EE: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14579

Development log

• July 10th: New Rubocop cop was announced on #backend and #development channels.