PLT-7599: webhook post splitting (#7707)

* webhook post splitting

* style fix

* update old webhook test

i18n/en.json

@@ -6860,8 +6860,8 @@
     "translation": "Back"
   },
   {
-    "id": "web.incoming_webhook.attachment.app_error",
-    "translation": "Maximum attachments length is {{.Max}} characters, received size is {{.Actual}}"
+    "id": "web.incoming_webhook.split_props_length.app_error",
+    "translation": "Unable to split webhook props into {{.Max}} character parts."
   },
   {
     "id": "web.incoming_webhook.channel.app_error",