Add organizations option to GitHub code host config

Created by: beyang

From a customer: But what I really wanted was to be able to just have a list of GitHub orgs, e.g.,:

https://github.com/envoyproxy
https://github.com/kubernetes
https://github.com/kubernetes-incubator

And have them benefit from things like initialRepositoryEnablement: true

We should support listing GitHub organizations as a way to specify which repositories to sync to a Sourcegraph instance. I think I recall another customer having this request, as well.