Fix duplicate repositories in GitHub import page

By default, all the current user's repositories are accessible via the
/users endpoint. There's no need to traverse all the organization
repositories as well.

See:

* http://www.rubydoc.info/github/pengwynn/octokit/Octokit/Client/Repositories#repositories-instance_method
* https://developer.github.com/v3/repos/#list-your-repositories

Closes #2523