Show members of parent groups on project members page

What does this MR do?

1. Show members of parent groups on project members page
2. Refactor Project members controller
3. Refactor and add specs for MembersFinder

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

no

Why was this MR needed?

To see members of parent groups on project members page

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/28786 and https://gitlab.com/gitlab-org/gitlab-ce/issues/27962