Webhook Secret

When creating a New GitHub Application via https://github.com/settings/apps/new
we are given the _option_ to add a `Webhook Secret`:
While the `Webhook Secret` is "_optional_", I feel it would add good "security layer" to our app.
Otherwise _anyone_ can "spoof" a webhook `POST` request to our app and make an "edit" to someone else's issue.

> Yes, this would be "non-destructive" because the "single-source-of-truth" is still _GitHub_.
But if the person made _multiple_ "malicious" edits they could create quite a lot of spam/noise.

I don't think we need to do this "urgently" while we are using the app _internally_,
but as soon as it's `public` we should consider adding this layer of protection.

How would this work in our Elixir/Phoenix App?
The `ruby` code in the docs: https://developer.github.com/webhooks/securing
should be _fairly_ easy to "translate" to Elixir.

Comienza leyendo la documentación de seguridad de webhooks de GitHub enlazada en la issue y, después, localiza el handler de POST del webhook de la aplicación Elixir/Phoenix. Determina cómo puede el handler validar el Webhook Secret de GitHub antes de aceptar modificaciones de issues; se considera completado cuando se rechazan las solicitudes falsificadas y los webhooks genuinos de GitHub siguen funcionando.

Escrito por el modelo de indexación a partir del texto del issue.