Add support for a single preview subdomain for PRs to main

[plaurent]

Allow content editors to see a live preview of their content/changes without requiring push to master.
Pull requests (targeting master) would be rendered to a preview site.

The subdomain of the preview versus the main URL, as well as their redirect URLs, should be configurable.

The Github Action would post a comment back on the pull request with the URL to which the site was published.

[plaurent]

Originally I considered having a separate subdomain for each PR # (somewhat similar to what Netlify does), but this would require us to automatically create install new callback URLs into the registered Azure AD application. (And that set up is manual for now.)