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.
Installing multiple copies of the same theme/component is possible, but you rarely need to actually have multiple copies installed. We've seen many times new admins installing duplicates of components because they were unaware it was already installed. This PR makes the theme installer modal loop through the existing themes when you click on 'install', and if there is a theme with a URL that matches the URL you entered, a warning will show up and you will need to click 'install' again to proceed.
