Replace uses of plugin-proxy.php with generic CORS proxy

[brandonpayton]

Now that we've started shipping a CORS proxy, let's move remaining uses of plugin-proxy.php to use the CORS proxy. Some remaining references are:

• For Core PR Preview
• For Gutenberg PR Preview

But there may be more.

[brandonpayton]

There is also documentation that refers to plugin-proxy.php, and we should update that as part of this issue.

[brandonpayton]

plugin-proxy.php has the ability to grab GitHub artifacts based on GitHub-specific IDs.

If we are going to replace plugin-proxy.php entirely, it will require folks integrating with Playground to provide the URL of the artifact on GitHub. This seems like an improvement because there is less that can break on the Playground side:

Either a download link works, or it does not.