Add Slack Webhook for new GHI and Daily CI

~~We are currently blocked from doing this b/c neither @kessplas nor @texastony have access to the webhook URL, but once that is addressed, we should make it so the Daily CI and updates to GHI triggers a slack notification.~~

The wonderful @rishav-karanjit has unblocked us!
#162 adds a webhook for Daily CI.

However, we should not add the Issue Notification wehbook until launch, as we do not want to spam the team channel with kessplass/texastony notifications.

首先查看 issue #162，因为它涵盖了 Daily CI webhook，并且可能会展示相关的集成点。这个 issue 剩余的工作是 GHI issue-notification webhook，该 webhook 只能在发布后添加，并且必须避免在发布前向团队频道发送垃圾信息。

由索引模型根据 Issue 内容生成。