New or changed manual webhook secrets (GitHub, GitLab, Bitbucket,
Gitea) must have at least 16 characters, in the Webhooks page and in
the application API. Stored secrets keep working: an unchanged value
passes, and an empty value still disables the webhook. Each secret
field gets a Generate button that fills a random 40-character secret;
the user saves it after adding it to the Git provider.
