Create Habari users for GitHub users when they send pings. Link them to ...

...their GitHub account so they can login with OAuth if socialadmin and githubauth are installed. Put them in a group and allow them to view addons.

The two plugins need to be present and active and githubauth needs to be configured with GitHub API keys to allow users to login with OAuth. Then just click "Login with GitHub" on the admin login page.

The code submitted in this pull request is not dependent on the plugins, but useless without them.