GitHub Actions: check labels

[maxim-belkin]

GitHub Action to check labels in the repository every Monday and on every push to gh-pages branch.

[fmichonneau]

maybe we could use something like this: https://github.com/micnncim/action-label-syncer ?

[maxim-belkin]

It'd be great to use an action for that but there is one unknown with this 'action-label-syncer' action: how does it handle changes to labels' colors and names? There is an (unanswered) issue with that question -- micnncim/action-label-syncer#44. Seeing the timeline on open issues/PRs, I'm a bit skeptical about using it but we can do that.

Regardless of whether we use this action or not, we need to decide how to handle labels:

1. Enforce labels according to a file in a lesson repository.
2. Enforce labels according to a file in some other (carpentries/style?) repository.

With (1) above, labels can get out of sync (unless we create a scheduled action to sync lessons with carpentries/styles)
With (2) above, we need to set up Personal Access Tokens for each lesson organization (SWC, DC, LC. Someone with admin privileges would have to do it).

And I also suggest we set up default labels for all four orgs (carpentries, swcarpentry, datacarpentry, librarycarpentry):
https://docs.github.com/en/github/setting-up-and-managing-organizations-and-teams/managing-default-labels-for-repositories-in-your-organization