Renaming `github_username` to `github_owner` + generating `__repo_name` & `__repo_url`

[matt-graham]

Though its not exposed to user now we have longer prompts rather than using variable names directly, I think github_username is a bit of a misnomer given that it can be either a user or organization name, hence I think github_owner would be a better choice.

We also currently manually construct the GitHub repository URL in lots of different bits of the template and also the qualified repository name in a few places. We can use double underscore prefixed (rendered) private variables in the cookiecutter config to generate the repository URL and qualified name once and then reuse elsewhere to avoid the repetition. This would also make it simpler to later switch to supporting alternative repository hosting options such as GitLab in future (which is the rational for naming the variables __repo_name and __repo_url rather than __github_repo_name and __github_repo_url).