A public key must be added to GitHub when installing private themes.
When the process happens asynchronously (for example if the admin does
not have admin permissions to the GitHub repository), installing
private themes becomes very difficult.
In this case, the Discourse admin can partially install the theme by
letting Discourse save the private key, create a placeholder theme and
give the admin a public key to be used as a deploy key. After the key
is installed, the admin can finish theme installation by pressing a
button on the theme page.
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