Fix post sort (#225)

Yet another wording confusion

Use `created_at` instead of `created` as post sort.

See non working sort https://www.data.gouv.fr/api/1/posts/?sort=-created
VS https://www.data.gouv.fr/api/1/posts/?sort=-created_at

Using a unknown field as sort leads to [undefined sort and duplicate
results](datagouv/data.gouv.fr#1072).

pages/beta/admin/site/posts.vue

@@ -107,7 +107,7 @@ const { t } = useI18n()
 
 const page = ref(1)
 const pageSize = ref(10)
-const sortedBy = ref<DiscussionSortedBy>('created')
+const sortedBy = ref<DiscussionSortedBy>('created_at')
 const direction = ref<SortDirection>('desc')
 const sortDirection = computed(() => `${direction.value === 'asc' ? '' : '-'}${sortedBy.value}`)
 const q = ref('')