Add Slack job status notifications to GitHub Actions CI

[mdickinson]

The GitHub actions CI (see #1287) does not currently have job notifications for Slack. If we ever want to replace Travis CI and Appveyor entirely with GitHub Actions, it might be useful to have those job notifications set up. (On the other hand, those notifications are only really valuable when cron jobs are set up, and we don't have any cron jobs for Traits.)

[rahulporuri]

(On the other hand, those notifications are only really valuable when cron jobs are set up, and we don't have any cron jobs for Traits.)

On that note, I wonder if/how cron jobs can be setup using github actions. Like you said, it's not relevant to traits but it needs to happen if we want to expand github actions to say envisage or traits-futures.

[mdickinson]

I wonder if/how cron jobs can be setup using github actions

https://docs.github.com/en/actions/reference/events-that-trigger-workflows#scheduled-events