Feature: Allow GH Teams to be created only with Identity Provider Groups

## Prerequisites:

* Is the functionality available in the GitHub UI? If so, please provide a link to information about the feature.

* Is the functionality available through the GitHub API? If the functionality is available, please provide links to the
API documentation (https://developer.github.com/v3/) as well as the Octokit documentation (https://octokit.github.io/).

* If the functionality is not yet available in the API, it would be helpful if you
contacted support (https://support.github.com/) or posted in the Community Forum (https://github.community/). Please
include a link to the forum post if you create one or a copy of the response from support.

## New Feature

Right now we have restricted the ability for repo members to create GH teams because we do not want them to make teams and add individual users. We want to have all teams mapped to an Identity Provider Group.

We would like a way to enforce using IdP groups when creating GH teams and disallowing ability to add individual users to a team manually.