Setup routes for accepting and declining invite

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https://gitlab.com/gitlab-org/gitlab/-/issues/372369

 accept_invite_group_group_member POST                  /groups/*group_id/-/group_members/:id/accept_invite(.:format)                                                                   groups/group_members#accept_invite

accept_invite_namespace_project_project_member POST                  /*namespace_id/:project_id/-/project_members/:id/accept_invite(.:format)                                                        projects/project_members#accept_invite

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