GitHub Actions: consider centralizing workflows

[zkamvar]

One of the main things that's currently painful in the lesson template is the way it's updated: via pull request.

I've been able to split away the components for updating it into two packages, {sandpaper} for the engine and {varnish} for the styling, but there is still a third, deployment component that are the github actions.

In out current mode of thinking, we are duplicating the actions across the repositories, but that's problematic because it means that they can be modified and we want to avoid doing that because we know that they will need to be updated in the future. This comment outlines the issue quite well: https://github.community/t/being-dry-centralized-workflows/16548

One solution is creating a stand-alone Composite action: https://docs.github.com/en/free-pro-team@latest/actions/creating-actions/creating-a-composite-run-steps-action

The requirement for this is to make sure that these actions do not require an extra secret to be added to the repos (or maybe that's not so bad with the tradeoff of having smaller actions).

[zkamvar]

I've started a test repository with a companion:

action: https://github.com/zkamvar/glowing-spork
test: https://github.com/zkamvar/symmetrical-spoon

Since the token for the checkout action persists, I am able to conditionally push to the repo. This means that it is possible for me to package most of the guts of the actions into separate repos.

[ErinBecker]

See #199