Navigate to GitHub

Users should be able to get from GitHub to ClearlyDefined, and also get from clearlydefined.io to the right place on GitHub. For example, when users are looking at a particular definition that has a curation or a contribution, they should be able to make an easy gesture and end up looking at the GitHub file changes related to that definition. There they should be able to do any of the normal GitHub operations such as comment on the change.