FEATURE: Add "Edit on Github" Button

[Alonski]

Would be cool to have a link at the bottom/top of a blog post with an Edit on GitHub link/button that links to the markdown and allows easy editing for creating PRs to fix mistakes :)

Related to:
https://github.com/ember-learn/guidemaker-ember-template/blob/master/app/templates/components/guides-article.hbs#L9 is essentially the same feature but for the guides
ember-learn/ember-website#23