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.

先阅读 issue 中链接的 GitHub webhook 安全文档，然后定位 Elixir/Phoenix 应用的 webhook POST 处理程序。确定处理程序如何在接受 issue 编辑之前验证 GitHub 的 Webhook Secret；当伪造请求被拒绝且真实的 GitHub webhook 仍能正常工作时，即表示完成。

由索引模型根据 Issue 内容生成。