preview support for repository templates

[shiftkey]

• https://developer.github.com/v3/repos/#create-a-repository-using-a-template
• https://developer.github.com/v3/repos/#create-a-repository-for-the-authenticated-user
• https://developer.github.com/v3/repos/#get-a-repository

This preview flag was not added in #2090 so repositories returned from the API do not have the is_template field set.

[github-actions]

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