Add pinned posts (#4217)

i18n/en.json

@@ -4623,6 +4623,10 @@
     "id": "store.sql_channel.extra_updated.app_error",
     "translation": "Problem updating members last updated time"
   },
+  {
+    "id": "store.sql_channel.pinned_posts.app_error",
+    "translation": "We couldn't find the pinned posts"
+  },
   {
     "id": "store.sql_channel.get.existing.app_error",
     "translation": "We couldn't find the existing channel"