Custom Webhook now adds a record in Webhook History table

[jaitjacob]

Custom Webhooks are not expected to send a UUID. Instead the UI checks for None passed for Webhook UUID and prints "Custom Webhook" instead. See below,

Fix #3455

[jaitjacob]

@FrostyX You may notice redundant db.session.commit() in webhook_general.py. I couldn't avoid them cause until we explicitly commit, 1.id is not generated and that results in an error

2. Webhook History table does not update with the latest webhook info

[praiskup]

Thank you very much for the quick turn-around. There's no need to send those simple fixes as drafts ;-)

[praiskup]

Thank you very much for the update; I'd just store NULLs for the user agent if not known, otherwise LGTM