Add discord ID to webhook notifications

[FlorianSW]

A discord user may leave the discord after they donated. This will invalidate the linked user reference in the discord notification. The name of the user is also added as plain text, however, the username can be changed by the user afterwards. To be able to uniquely identify the discord user (e.g. in mod-logs or similar), the discord ID is required. This should be added to the notification, just like the steam ID as well.