Secret should be removed from webhook body by default

[guillep2k]

Gitea version: 1.10.0-dev 3810fa4

Since header signatures were introduced in #6428, I think the default behavior should be to remove the secret from the body of the webhook request, as it is exposed in plain text and defeats the purpose of the signature. It's usually not a problem when the connection is https, but many struggle to configure that kind of setup, especially on internal servers that can't get a Letsencrypt certificate.

gitea/models/webhook.go

Line 739 in 3810fa4

p.SetSecret(w.Secret)

I know this is a breaking change, so I propose an app.ini setting and default it to not include the secret in the body. The admin can change it if they need the old behavior. An ini setting will not be enough if the users need by-repo configuration, though.