Implement some channel endpoints for APIv4 (#5767)

i18n/en.json

@@ -35,6 +35,10 @@
     "id": "June",
     "translation": "June"
   },
+  {
+    "id": "store.sql_channel.get_unread.app_error",
+    "translation": "We couldn't get the channel unread messages"
+  },
   {
     "id": "March",
     "translation": "March"