Revamp our live deployment and dev preview model and workflows

PR Goal?

Overhaul the way we deploy the Studio-Web app to https://readalong-studio.mothertongues.org/

Merging a PR to deploy will now update the live version on https://readalong-studio.mothertongues.org/
Pushing to main (or merging a PR) will now update the dev preview version at https://readalong-studio.mothertongues.org/dev/
Creating or updating a PR will continue to deploy PR previews at https://readalong-studio.mothertongues.org/pr-preview/pr-NNN

This is not quite what we agreed on: we were going to deploy from main and preview from dev, but I decided deploy and main made more sense because in every project, main is our development branch, and I think it would have caused frequent confusion to change that model here.

Still to do (future PR): make the live deployment use the published bundle, in sync with npm.

Note: I created the deploy branch and pointed it at the last commit that uses web-component 1.3.0 unmodified. When we merge this PR to deploy, the live deployment will roll back to not have the gear menu, which will only be visible on the dev preview for now, until we publish 1.4.0 and re-deploy.

Feedback sought?

Feedback on the model itself and my choice of branch names. Workflow code feedback.

Priority?

high

Tests added?

This can't be tested automatically, but I tested by running these workflows on my fork. See

https://joanise.github.io/Studio-Web/pr-preview/pr-280/ (PR preview)
https://joanise.github.io/Studio-Web/dev/ (dev preview)
https://joanise.github.io/Studio-Web/ (live deployment) These don't work because I'm block by CORS from the API, but if you look at the network traffic you'll see it's loading what we expect, and if you look at the gh-pages branch on https://github.com/joanise/Studio-Web you'll see the expected structure.

I have validated that main, dev and pr-previews don't interfere with each other.

How to test?

No need, unless you want to redo my fork tests on your own fork.

Confidence?

High

Version change?

nope

ReadAlongs / Studio-Web