Add alerts count badge to alerts tabs

...pts/alert_management/components/alert_management_list.vue

..._management/graphql/fragments/detailItem.fragment.graphql

...t_management/graphql/fragments/list_item.fragment.graphql

...alert_management/graphql/queries/get_alerts.query.graphql

...agement/graphql/queries/get_count_by_status.query.graphql

app/controllers/projects/alert_management_controller.rb

changelogs/unreleased/217670-alerts-count.yml

...alert_management/components/alert_management_list_spec.js