Add authentication for admin page

We need to control access to the admin page so that only administrators can change settings. It will also help us to keep track of who made the change in the Admin page.

We can use local authentication or GitHub authentication
http://www.passportjs.org/packages/passport-github/

The issue identifies the admin page and links to Passport's GitHub authentication package, but names no source files, tests, or entry points. Start by locating the admin page and deciding between local and GitHub authentication; done should restrict settings changes to administrators and record who made each change.

Written by the indexing model from the issue text.