Add feature flag for allow bypass of placeholder user confirmation

• Add wip feature flag importer_user_mapping_allow_bypass_of_confirmation. Scoped to @group.
• Push the flag to frontend in https://gitlab.com/gitlab-org/gitlab/blob/81490c55cd989e676da14104c3e36643d1a69ac0/app/controllers/groups/group_members_controller.rb#L21.