Add support for conflicts to issuable_item.vue

Each issuable item in an issuable list app uses the https://gitlab.com/gitlab-org/gitlab/-/blob/b7511a8d301e12a71588b76ecba2606f98b60ed4/app/assets/javascripts/vue_shared/issuable/list/components/issuable_item.vue component. We need to update this component to support the conflicts badge