Add support for repository has_discussions

[rgmz]

Prerequisites:

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

The GitHub UI settings allows you toggle features including: Wikis, Issues, Discussions, and Projects.

• 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/).

The Create an organization repository endpoint has fields for has_issues, has_projects, has_wiki, and has_downloads but does not have a has_discussions field. However, Create a repository for the authenticated user does.

has_discussions is returned from the Get a repository endpoint.

• 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

Please describe the desired new functionality:

It would be nice if Discussions could be programmatically enabled/disabled in the repository settings, similar to has_issues, has_projects, and has_wiki.

It isn't immediately obvious to me whether Discussions are supported in the API, similar to the aforementioned options. Perhaps this is a known limitation and my attempts to search for previous issues failed me?