[Rails5] Fix GROUP BY for mysql

What does this MR do?

Fixes issue with "SELECT list is not in GROUP BY clause and contains nonaggregated column" error for mysql.

Similar to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20256

Are there points in the code the reviewer needs to double check?

Yes, this changes SQL query with potential performance impact.

Why was this MR needed?

Because only_full_group_by is default in rails 5, we must have all columns from SELECT also in GROUP BY clause.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• EE specific content should be in the top level /ee folder
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #6932 (closed)