Add Generic Webhook as an option on Notification Settings

[alnviana]

Basically, I have a local webhook that I want to use, but there isn't a option to be chosen. It's possible to ignore the Service Type and put the "generic" type straight on the URL, save it and the Notification arrive. But if you try to edit that channel afterwards, isn't possible to save, because Netronome doesn't recognize "generic" as an valid type.

So I think it's just necessary to put the "generic" option and recognize it as valid, as shoutrrr supports it.