Support backuping up API data

To provide a full backup of the GitHub repository, we want to also get project data such as the wiki, issues, milestones, etc. This series reworks the updater so that it has the notion of backing up data from the GitHub API in addition to git repositories. The format of the backup is very simple: the content type (e.g., issues) is appended to the repository name, and then the returned API data is stored with paths relative to the repository API url. The API endpoints supported are just those needed for Endless backups, but more can be easily added by subclassing JsonRepo.

[endlessm/eos-shell#2038]