Backport #17482
* Only allow webhook to send requests to allowed hosts (backport #17482)
* use ALLOWED_HOST_LIST=* for default to keep the legacy behavior in 1.15.x
We allow to render empty check list item - [ ], while GitHub doesn't allow.
To make the rendering correct, we need tune the UI (the last PR #17413 uses absolute layout, which makes the empty checkbox item can not be displayed correctly)