Send Slack Notification on Deployment

[shawnhooper]

Fixes #77

To test this:

1. Create a new Slack app. Copy it's OAuth Access token.

2. Create a new GitHub secret with the OAuth Access token.

3. Invite the Slack app into the channel where you want to announce the deployments.

4. Add two new environment variables to the preview-provision.yml workflow:

     SLACK_ANNOUNCEMENT_ENABLED: true
     SLACK_BOT_USER_OAUTH_TOKEN: ${{ secrets.SLACK_BOT_USER_OAUTH_TOKEN }}
     SLACK_CHANNEL: "#deployments"  (replace this with the channel you invited the bot to)

5. Test a deployment.

The Slack Notification will only be sent when the site is first created. Subsequent pushes to the same PR won't trigger the notification.

[shawnhooper]

This is proof of it working in my environment

[shawnhooper]

Thanks @mehrancodes !