Refactor code that creates and destroys project/group members

What does this MR do?

• Reuse members create service for group controller
• Move create member controller method to concern
• Move related tests from spinach to rspec

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

no

Why was this MR needed?

Refactoring, cleanup

Screenshots (if relevant)

no

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support 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?

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/14709