Add slack-github-action

[rickycodes]

This will announce when a workflow run needs review/approval on Slack

TODO:

• [ ] add slackapi/[email protected] to org list of approved GitHub Actions
• [ ] add secrets.SLACK_BOT_TOKEN to repository (who do I talk to about this?)
• [ ] consider maybe adding this as a config option to action-npm-publish so we don't have to paste this block everywhere?

re: https://github.com/MetaMask/action-npm-publish/issues/6