MM-11172: Don't allow reacting in read-only town square. (#9106)

i18n/en.json

@@ -159,6 +159,10 @@
     "id": "api.channel.delete_channel.deleted.app_error",
     "translation": "The channel has been archived or deleted"
   },
+  {
+    "id": "api.reaction.town_square_read_only",
+    "translation": "Reacting to posts is not possible in read-only channels."
+  },
   {
     "id": "api.channel.delete_channel.type.invalid",
     "translation": "Cannot delete direct or group message channels"