Refactor Admin::GroupsController#members_update method and add some specs

What does this MR do?

1. Use Members::CreateService instead of add_users for Admin::GroupsController#members_update method
2. Add specs

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

no

Why was this MR needed?

1. Improve code quality
2. Show users and error when no users added

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