Support configuring default branch name

[ijlee2]

Description

The project assumed that the default branch name of a GitHub repo is master:

https://github.com/ember-learn/guidemaker-ember-template/blob/0245119e8ce524b568cb259f13308d03135157b7/app/templates/components/guides-article.hbs#L11

When we update the default branch name to main, clicking on the edit link in the consuming app will lead to a 404 page on GitHub. (GitHub currently supports redirecting view links only, and not edit links.)

Related issue: https://github.com/ember-learn/cli-guides/issues/228

Possible solutions

• Allow passing a configuration parameter for the default branch name
• Make a major release that assumes main as the default branch
• Something else