Add backend infrastructure for Extras

This pull request adds the necessary backend infrastructure for the Extras view. Notably it adds:

- The necessary routing plumbing.
- A `githubApi` module responsible for querying with Github. The module only queries for new repos if a certain time threshold has been crossed. It stores the results in memory. 
- `extras` route handler that has an `inProgress` flag which checks to see if an API call is in progress, and serves the older out-of-date result set while the new set completes.
- `extras.handlebars` view that loops through the returned Github repo information and creates the necessary DOM.