Refresh a form or event preview graphic

Extra step for Telegram:

If a url has been shared previously the preview is cached and will not update automatically. To force a fresh link preview, 3 options:

1. shorten the url
2. change the url by adding dummy parameters at the end (?V=1 say)
3. go to https://t.me/WebpageBot and submit the url to force a refresh across Telegram