Ensure approver references are all present

What does this MR do?

Fix one bug of data migration in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8669 where an approver association is pointing to an no longer existing user_id.

When assigning using _ids =, the ids provided must all be present. However some data in approvers table are users which has been destroyed. Therefore Rails would raise exception.

The fix does extra checking to ensure user_ids and group_ids are all still present.

What are the relevant issue numbers?

#8647 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer