GitHub urls should be listed beneath the github field

New structure for the github url, we list them beneath the `github` field so that we can loop through them. We will likely make authors work the same way, eventually.

In the YAML we now list out the GitHub URLs like this:
```
github:
- https://github.com/18f/dashboard
- https://github.com/18f/18f.gsa.gov
```
Even if there's only one repo. Then in the template we assume the first one listed is the primary repo for that project and make the :octocat: icon link and issues, stars, etc. for that repo