Automatically enable branch protection

[Apreche]

Djangogoboot sets up a new project that has two GitHub actions for checks. One for lint, and one for tests. However, by default users can just go ahead and merge PRs that do not pass these checks. GitHub's protected branch feature must be used to actually require that PRs pass checks before being merged to main.

The problem is that only paying GitHub subscribers can protect branches on private repositories. We want to automatically set main to be a protected branch that requires checks to pass, but we must first detect if the GitHub user is a subscriber or if the repository is public (see issue #1).