Ensure that Sourcegraph instances don't overwrite BBS webhook

Created by: mrnugget

If you have multiple Sourcegraph instances running and configured to use the "plugin.webhooks" attribute, they would overwrite each others webhooks because they used the same name.

This change here attaches the unique SiteID to each webhook. It doesn't change across restarts and persists between changes of external URLs, so it's perfect for our usecase.