Add rubocop gem

What does this MR do?

This adds the rubocop gem and configuration modified from the one in Omnibus.

It does not:

• turn on rubocop in the pipelines
• make it default

This is just to add it so we can start the cleanup work and verify that we agree with the configuration.

Related issues

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened

Closes #2426 (closed)