Set organization default member permissions through API if possible

[pconrad]

Organization member privileges are settable through the settings menu.

The "correct" for a github organization used for a course is that under member privileges, the "Default repository permission" should be set to "None", i.e.

Members will only be able to clone and pull public repositories. To give a member additional access, you’ll need to add them to teams or make them collaborators on individual repositories.

if we can do this through the API, we should consider doing it.

Or maybe we just check it, and give admin users warnings if they permission is set to one that we don't expect? And advise them to change it themselves through the github web interface...

Also, there are two other permissions that we may want to check and/or modify as well.